Judgment vacated and new trial awarded by published opinion. Judge KING announced the judgment of the Court, in which Judge WIDENER, Judge WILKINS, Judge LUTTIG, Judge WILLIAMS, Judge MICHAEL, Judge DIANA GRIBBON MOTZ, and Judge TRAXLER joined; wrote the opinion of the Court with respect to Part III, in which Judge WILKINS, Judge WILLIAMS, Judge MICHAEL, Judge DIANA GRIBBON MOTZ, and Judge TRAXLER joined; wrote the opinion of the Court with respect to Parts IV and V, in which Judge WIDENER, Judge WILKINS, Judge LUTTIG, Judge WILLIAMS, Judge MICHAEL, Judge DIANA GRIBBON MOTZ and Judge TRAXLER joined; and wrote an opinion with respect to Parts I and II in which Judge WIDENER (except perhaps for a footnote), Judge LUTTIG (in part), Judge MICHAEL, and Judge DIANA GRIBBON MOTZ joined. Judge WIDENER wrote an opinion concurring in part and concurring in the judgment. Judge WILKINS wrote an opinion concurring in part and concurring in the judgment, in which Judge WILLIAMS and Judge TRAXLER joined. Judge LUTTIG wrote an opinion concurring in part and concurring in the judgment. Chief Judge WILKINSON wrote a dissenting opinion, in which Judge NIEMEYER joined. Judge NIEMEYER wrote a dissenting opinion, in which Chief Judge WILKINSON joined and in which Judge TRAXLER joined with respect to Parts I and II.
OPINION
KING, Circuit Judge:
Michael Rhynes and several co-defendants were tried before a jury in the Western District of North Carolina on a number of drug-related charges. During the presentation of Rhynes’s defense, the district court excluded his sole supporting witness after finding that his lawyer had violated the court’s sequestration order. We conclude today that the exclusion of the witness’s testimony was improper and constitutes reversible error. The conduct of Rhynes’s lawyer did not contravene the district court’s sequestration order, and, if it had, the sanction of witness exclusion was unduly severe. Because this error was not harmless, we must vacate Rhynes’s conviction and sentence and remand for a new trial.
I.
A.
At the outset, we briefly review the proceedings that have brought us to en banc review. The convictions of Michael Rhynes and his six co-defendants followed a three-week trial, and a panel of this Court subsequently heard and considered their consolidated appeals. On October 26, 1999, the panel disposed of the appeals [313]*313by a published decision that, inter alia, affirmed Michael Rhynes’s conviction and thirty-year sentence thereon. See United States v. Rhynes, 196 F.3d 207, 243 (4th Cir.1999).
Thereafter, each of the defendants petitioned for rehearing, and the Government cross-petitioned, seeking rehearing of certain issues decided in the defendants’ favor. On February 3, 2000, we denied, with a single exception, each of the rehearing petitions. In the exception, we deconsoli-dated Rhynes’s appeals from those of his co-defendants and granted rehearing en banc on a single issue: whether the district court’s exclusion of Corwin Alexander as a witness constitutes reversible error (the “witness exclusion issue”). By granting limited rehearing en banc, we vacated the panel decision insofar as it relates to the witness exclusion issue. See Local Rule 35(c). On April 4, 2000, that issue was argued before the en banc court.
B.
The panel opinion thoroughly recounts the extensive and complicated history underlying the convictions of Michael Rhynes and his co-defendants. Rhynes, 196 F.3d at 213^43. Thus, we focus here only on the facts relating to the witness exclusion issue.
1.
On September 24, 1996, at the commencement of the trial in Charlotte, North Carolina, a lawyer for one of Rhynes’s co-defendants moved for sequestration of the Government’s witnesses. In response, the district court entered its sequestration order from the bench.1 The Government then noted that its “case agent” and a “summary witness” were in the courtroom and intended to “sit[ ] in on the testimony prepared to testify at the end of the trialt.]” J.A. 274. The district court granted the Government’s request that two of its witnesses be excepted from the sequestration order and another motion that the defense witnesses be sequestered. Thereafter, the lawyer for one of Rhynes’s co-defendants sought to have his investigator excepted from the sequestration order, and the court granted the exception “[s]o long as your investigator observes Rule 615 and does not talk to the witnesses about testimony that has just concluded or testimony that has concluded.” J.A. 275.
2.
During the Government’s case-in-chief, it presented the testimony of witness D.S. Davis. Davis is a convicted felon and was, at the time of trial, serving a seven-year sentence for participating in a drug conspiracy. Davis testified, inter alia, that he first met Alexander in 1990, when he (Davis) asked Alexander to serve as an intermediary in a drug transaction between Davis and Michael Rhynes.
In response to an objection from Rhynes’s lawyer, Michael Scofield, the Government explained at the bench that it was “getting to the focal point of Mike Rhynes.” J.A. 1695. Specifically, the Government noted that Alexander was Ust-ed as a witness for Rhynes, and it believed that Davis would testify that Alexander had been approached to serve as an intermediary between Davis and Rhynes but that Alexander had never completed a transaction between the two. In response, Mr. Scofield stated:
Well, I didn’t know what — where he was going with Corwin Alexander, so I had [314]*314no information that they allege he was a member of the conspiracy. It sounded like you’re not alleging that now.
J.A. 1695. The Government replied, apparently in reliance on Davis’s statements, that Alexander had told Davis that Rhynes “did not have the drugs at that time,” J.A. 1696, and Davis thereafter dealt directly with Rhynes.
Davis then concluded his testimony in support of the Government’s case. He maintained that Alexander had approached' Rhynes on two occasions with proposals of drug deals with Davis. According to Davis, Alexander passed those offers to Rhynes, who did not accept either offer. Subsequently, Davis approached Rhynes directly, and Rhynes agreed to sell him cocaine. Thereafter, according to Davis, Rhynes sold him drugs on several occasions.
3.
During Rhynes’s defense, he testified on his own behalf; then, he called a single witness to corroborate his testimony: Cor-win Alexander. Alexander testified on a number of subjects, see infra note 3, before he was asked about the Government’s earlier witness, Davis. Alexander explained that, at a meeting between the two, Davis told Alexander that the Government had offered Davis a deal in exchange for information about Rhynes. Alexander then stated, “And he [ (Davis) ] went off to do his time, and I hear from Tuesday he got up and said — ,” whereupon the Government objected and requested a bench conference. J.A. 1945L.
At the bench, Mr. Scofield advised the district court that he had discussed Davis’s testimony with Alexander: “I specifically told him about that testimony and told him I was going to ask him about that, Your Honor. And I don’t think that violates the sequestration order.” J.A. 1945M. The district court indicated its belief that the sequestration order had been violated. Mr. Scofield then responded, “I’m sorry then, Your Honor. I’ve done wrong then because I don’t know how else I can prepare him to testify. I told him that that guy told him that he was a drug dealer.” Id.
The district court nonetheless granted the Government’s motion to strike Alexander’s testimony and to exclude him as a witness.2 The court did not develop the record further, either by obtaining any additional testimony from Alexander or by securing a proffer or testimony from Mr. Scofield.
4.
Following a brief recess, which apparently included an off-the-record in camera discussion, Mr. Scofield requested another bench conference to discuss the witness exclusion issue. There, he apologized to the court and attempted to deflect any sanction away from his client and onto himself, stating:
Your Honor, as I told you in chambers, I now realize that the proper thing for me to do in interviewing Alexander and preparing him to testify was that I could have asked him all the details of whether he had been a dealer and whether he had done drug deals with Michael Rhynes and that sort of thing without telling him that Davis had said that he had done that.
I wanted to specifically ask him about his relationship with D.S. Davis. And as I told the court, I did tell him that D.S. Davis had said that he had done these drug deals and that I wish I had been more alert in drawing that line about just asking the questions without saying what D.S. Davis had said in the court. I [315]*315am concerned that my mistake will rebound to the harm of my client and my client will be prejudiced and I would— I’d asked the court in chambers if you would revisit your decision to strike Cor-win[ ] [Alexander’s] testimony and not let him testify further. The court indicated that it would not, and I asked if I could make a proffer.
J.A.1945P-45Q (emphasis added).
Scofield then made a proffer of the balance of Alexander’s testimony.3 According to Mr. Scofield, Alexander would have testified that he never dealt drugs with Davis or anyone else. Alexander also knew Davis well enough to form the opinion that Davis was untruthful, and that Davis was testifying under pressure to “save three years” of imprisonment. J.A. 1945R. Moreover, Alexander would have corroborated Rhynes’s testimony about an automobile accident and an insurance settlement, which would provide further explanation for what the Government had presented as Rhynes’s “unexplained wealth.” Alexander also would have challenged the testimony of another witness, Ted Howze, about Howze’s dealings with Rhynes, and Alexander would have offered both his opinion and reputation evidence that other witnesses, including Andy Stin-son, Kenny Funderburk, Lester Norman, Jerry Harrison, and Tyron Hicks were untruthful. Further, Alexander would have testified that he accompanied Rhynes on a trip to New York, which the Government claimed had been a drug-related trip. Alexander would have explained that he and Rhynes had gone to see a basketball game and that they had had no involvement with drugs, on this trip or otherwise. Finally, Alexander would have corroborated other details about Rhynes.
At the conclusion of the bench conference, the district court denied Mr. Sco-field’s request to revisit the witness exclusion issue,4 and Alexander’s testimony was excluded from the trial.
II.
We review a district court’s evidentiary rulings, including an.order excluding witnesses or striking their testimony, for an abuse of discretion. General Elec. Co. v. Joiner, 522 U.S. 136, 141-42, 118 S.Ct. 512, 139 L.Ed.2d 508 (1997). Alternatively, we review de novo a district court’s resolution of questions of law. United States v. Legree, 205 F.3d 724, 728 (4th Cir.2000).
[316]*316B.
It is significant to our consideration of this appeal that the district court invoked “the usual sequestration rule ... that the witnesses shall not discuss one with the other their testimony.” This order thus had two parts: (1) the “usual sequestration rule” and (2) that witnesses were not to discuss their testimony with one another (“extending language”).
For several reasons, it is apparent that the reference to “the usual sequestration rule” did nothing more than invoke Federal Rule of Evidence 615, the rule relating to sequestration orders. At the outset, the reference to a “rule” in the order was an obvious invocation of Rule 615.5 The district court’s subsequent statements relating to the sequestration order similarly made clear to each of the parties that “the usual sequestration rule” was coextensive and conterminous with Rule 615. First, immediately after entering its order, the court permitted a co-defendant’s investigator to remain in the courtroom, but only if the investigator “observes Rule 615.” J.A. 275. Later, when it initially interrupted Alexander’s testimony, the court stated, “It’s an absolute breach of the Rule 615” (J.A. 1945M); similarly, when the court decided not to revisit its witness exclusion determination, it noted: “[M]y Rule 615 order was violated as to the testimony of many witnesses.” J.A. 1945U. Given these three clear references to Rule 615, we can only conclude that the district court’s reference to a “rule” did nothing more than invoke Rule 615.
Because the district court’s order involved these two elements, our task here is to first ascertain whether either (1) Rule 615 or (2) the additional admonition that “the witnesses shall not discuss one with the other them testimony” proscribed the conversation Mr. Scofield had with Alexander. We begin with a review of Rule 615 — an analysis that we undertake de novo.
C.
To determine whether Rule 615 proscribes the conduct of Mr. Scofield, we must first consider the language of the Rule itself, which provides:
At the request of a party the court shall order witnesses excluded so that they cannot hear the testimony of other witnesses, and it may make the order of its own motion. This rule does not authorize exclusion of (1) a party who is a natural person, or (2) an officer or employee of a party which is not a natural person designated as its representative by its attorney, or (3) a person whose presence is shown by a party to be essential to the presentation of the party’s cause, or (4) a person authorized by statute to be present.
Fed.R.Evid. 615. It is clear from the plain and unambiguous language of Rule 615 that lawyers are simply not subject to the Rule. This Rule’s plain language relates only to “witnesses,” and it serves only to exclude witnesses from the courtroom. Thus, Rule 615 did not prohibit Mr. Sco-field from discussing D.S. Davis’s testimony with Corwin Alexander.6
[317]*317The district court’s, bald Rule 615 order was then extended by the statement that “the witnesses shall not discuss one with the other their testimony.” Of course, nothing on the face of this extending language addresses the conduct of lawyers in any way. Moreover, the relevant authorities interpreting Rule 615, including court decisions and the leading commentators, agree that sequestration orders prohibiting discussions between witnesses should, and do, permit witnesses to discuss the case with counsel for either party: “Sequestration requires that witnesses not discuss the case among themselves or anyone else, other than the counsel for the parties.” United States v. Walker, 613 F.2d 1349, 1354 (5th Cir.1980) (emphasis added) (citing Gregory v. United States, 369 F.2d 185 (D.C.Cir.1966)); accord United States v. Buchanan, 787 F.2d 477, 485 (10th Cir.1986) (“The witnesses should be clearly directed, when [Rule 615] is invoked ... that they are not to discuss the case ... with anyone other than counsel for either side”) (emphasis added); 4 Jack B. Weinstein & Margaret A. Berger, Weinstein’s Federal Evidence § 615.06 (Joseph M. McLaughlin, ed., 2d ed. 1998) (“[Sequestration] instructions, however, usually permit the witnesses to discuss their own or other witnesses’ testimony with counsel for either side .”) (emphasis added); 2 Charles A. Wright, Federal Practice & Procedure § 415 (2d ed. 1982) (“If exclusion is ordered, the witnesses should be instructed not to discuss the case with anyone except counsel for either side.”) (emphasis added).7
In short, neither the bald invocation of Rule 615 nor the extending language relating to discussions between witnesses served to circumscribe the conduct of Mr. Scofield in any way.
D.
The Government has conceded that neither the plain language of the district court’s order, nor the provisions of Rule 615, prohibit any conduct by lawyers, and we note that “in all but the most extraordinary circumstance,” the inquiry should end here. Estate of Cowart v. Nicklos Drilling Co., 505 U.S. 469, 475, 112 S.Ct. 2589, 120 L.Ed.2d 379 (1992). Nonetheless, the Government asserts that. the “purpose and spirit” of the sequestration order were compromised by Mr. Scofield’s discussion with Alexander. Specifically, the Government contends that the “truth-seeking” process would be Hindered if lawyers were permitted to reveal testimony in the manner exercised by Mr. Scofield. This is basically an argument that Rule 615, the extending language, or the policies underlying sequestration implicitly proscribed Mr. Scofield’s conduct; we reject the argument for several reasons.
We have properly recognized the purpose and spirit underlying witness sequestration: it is “designed to discourage and expose fabrication, inaccuracy, and collusion.” Opus 3 Ltd. v. Heritage Park, Inc., 91 F.3d 625, 628 (4th Cir.1996). Put differently, sequestration helps to smoke out lying witnesses: “It is now well recognized that sequestering witnesses' ‘is (next to cross-examination) one of the greatest engines that the skill of man has ever invented for the detection of liars in a court of [318]*318justice.’ ” Id. (citing 6 Wigmore on Evidence § 1838, at 463).
To the extent that the Government asserts that Mr. Scofield frustrated the purpose and spirit of sequestration, we disagree. The Government asserts that Mr. Scofield’s actions undermined the truthfulness of Alexander’s testimony, which, in the Government’s view, is surely an act that runs afoul of the sequestration order. On the contrary, lawyers are not like witnesses, and there are critical differences between them that are dispositive in this case. Unlike witnesses, lawyers are officers of the court, and, as such, they owe the court a duty of candor, Model Rules of Professional Conduct Rule 3.3 (1995) (“Model Rules”). Of paramount importance here, that duty both forbids an attorney from knowingly presenting perjured testimony and permits the attorney to refuse to offer evidence he or she reasonably believes is false. Id. Rule 3.3(a)(4), (c). Similarly, an attorney may not “counsel or assist a witness to testify falsely.” Id. Rule 3.4(b). And, if an attorney believes that a non-client witness is lying on the witness stand about a material issue, he is obliged to “promptly reveal the fraud to the court.” Id. Rule 3.3, cmt. 4. The Supreme Court has emphasized the importance of attorneys’ duty of candor: “Any violation of these strictures would constitute a most serious breach of the attorney’s duty to the court, to be treated accordingly.” Geders v. United States, 425 U.S. 80, 90 n. 3, 96 S.Ct. 1330, 47 L.Ed.2d 592 (1976) (citing to parallel provisions of Model Code of Professional Responsibility ). Consequently, lawyers’ ethical obligations to the court distinguish them from trial witnesses.
Moreover, the purpose and spirit underlying sequestration are not absolute; indeed, we have aptly recognized that even the “powerful policies behind sequestration” must bend to the dictates of the Constitution. Opus 3 Ltd., 91 F.3d at 628.8 Thus, to the extent that they are implicated in this case, the policies and spirit of sequestration must yield to the constitutional and ethical duties Mr. Scofield sought to effectuate here.9 That is, in the context of a criminal trial like this one, a defense attorney’s duty to his client assumes constitutional stature: “In all criminal prosecutions, the accused shall ... [319]*319have the Assistance of Counsel for his defence.” U.S. Const, amend. VI. To all clients, an attorney owes competence. Model Rules Rule 1.1. To fulfill this basic duty, the attorney must prepare carefully for the task at hand: “Competent representation requires ... thoroughness and preparation reasonably necessary for the representation.” Id. Rule 1.1(a).
Thorough preparation demands that an attorney interview and prepare witnesses before they testify. No competent lawyer would call a witness without appropriate and thorough pre-trial interviews and discussion. In fact, more than one lawyer has been punished, found ineffective, or even disbarred for incompetent representation that included failure to prepare or interview witnesses. United States v. Tucker, 716 F.2d 576 (9th Cir.1983) (defense counsel ineffective for failing to interview witnesses); McQueen v. Swenson, 498 F.2d 207 (8th Cir.1974) (same); In re Warmington, 212 Wis.2d 657, 668, 568 N.W.2d 641 (1997) (lawyer disbarred for, among other things, “failing to supervise the preparation of an expert witness”); In re Wolfram, 174 Ariz. 49, 847 P.2d 94, 96 (1998) (failure to interview witnesses cited among reasons for suspending attorney).
In this context, Mr. Scofield’s actions were necessary in the exercise of his duties, both constitutional and ethical, as a lawyer. First, when the Government called Davis as a witness and began asking him questions about Alexander, Mr. Sco-field made clear that he was unaware that Alexander had been implicated as a co-conspirator. See supra at 313-14. Although Davis’s subsequent testimony did not implicate Alexander in any specific drug deal, the import of Davis’s allegation was clear: Alexander was serving as an intermediary between drug buyers and Rhynes. Faced with an allegation that his prime supporting witness, Alexander, had been assisting, or participating in, a drug conspiracy with Rhynes, Mr. Scofield had ethical (and possibly constitutional) duties to investigate these allegations with Alexander before he put Alexander on the stand. Mr. Scofield was thus compelled to ascertain, if possible: (1) whether Davis’s allegations were untrue (or, if true, whether Alexander intended to invoke his Fifth Amendment rights); (2) whether Alexander’s denials were credible; and (3) why Davis would make potentially false allegations against Alexander. Put simply, Mr. Scofield needed to fully assess his decision to call Alexander as a witness, and, to fulfill his obligations to his client, Scofield was compelled to discuss Davis’s testimony with Alexander. See Chandler v. Jones, 813 F.2d 773 (6th Cir.1987) (finding counsel’s performance deficient for (1) failing to prepare witness for trial; (2) improperly using leading questions; and (3) calling witness who was expected to invoke the Fifth Amendment).
In response, the Government claims that Mr. Scofield did not violate the sequestration order by merely speaking with Alexander; instead, it was Mr. Scofield’s informing Alexander of Davis’s testimony that violated the order. Based on this view, the Government asserts that counsel had ample room to interview and prepare witnesses without running afoul of the sequestration order.10 But this conclusion [320]*320begs the question, “How was counsel to discern the limits of the sequestration order?” Those limits — as declared after the fact by the district court — did not appear on the face of the order, in Rule 615, in controlling precedent, or even in persuasive authorities. In fact, adoption of the Government’s position would make it virtually impossible for counsel to know whether they have “ample room” to perform essential tasks without violating an order.11 This argument thus fails to persuade us.
Further, sequestration is not the only technique utilized to ensure the pursuit of truth at trial. Indeed, if an attorney has inappropriately “coached” a witness, thorough cross-examination of that witness violates no privilege and is entirely appropriate and sufficient to address the issue. In Geders, Chief Justice Burger, for a unanimous Court, endorsed cross-examination as the swift antidote for witness coaching:
The opposing counsel in the adversary system is not without weapons to cope with “coached” witnesses. A prosecutor may cross-examine a defendant as to the extent of any “coaching”.... Skillful cross-examination could develop a record which the prosecutor in closing argument might well exploit by raising questions as to the defendant’s credibility, if it developed that defense counsel had in fact coached the witness as to how to respond....
Geders, 425 U.S. at 89-90, 96 S.Ct. 1330.
In short, the Government’s position requires the implication that by discussing prior trial testimony with Corwin Alexander, Mr. Scofield necessarily coached Alexander or made it likely that Alexander would commit perjury.12 To the contrary, we must trust and rely on lawyers’ abilities to discharge their ethical obligations, including their duty of candor to the court, without being policed by overbroad sequestration orders. Furthermore, we are confident that, if an attorney is lax in his duty of candor, that laxness will normally be exposed — even exploited — by skillful cross-examination.
E.
Undeterred by the weight of contrary authority, the Government asserts that the district court’s ruling that its order actually prohibited attorney-witness discussions of testimony is a permissible “interpretation” of the sequestration order. However, it is apparent that the district court was not interpreting its own order; rather, it was interpreting Rule 615. That is, the extending language of the sequestration order does not in any way relate to attorneys, and in each of the post-entry statements relating to the violation of its sequestration order, the district court clearly believed Rule 615 — not the extending language in its order — had been violated. See supra at 316-17. In this context, reliance by the Government and my dissenting colleagues on the inherent discretion of a presiding judge is a red herring. We simply do not defer to a district court’s legal interpretation of federal rules; those interpretations are reviewed de novo.
Further, the Government cites no authority for the proposition that an unadorned sequestration order, devoid of any reference to lawyers, nevertheless may be interpreted to prohibit lawyers from discussing trial proceedings with prospective [321]*321witnesses. More importantly, if a district court does not exceed its discretion by interpreting a sequestration order in a manner that: (1) is unsupported by its text; (2) is unsupported by Rule 615; (3) is unprecedented in this circuit; (4) is contrary to the overwhelming weight of persuasive case law and scholarship; and (5) arguably unconstitutionally deprived the defendant of effective assistance of counsel, then the district court’s discretion to interpret its orders is effectively limitless. As a practical matter, our adoption of the Government’s position would permit trial judges, when faced with any trial activity they dislike, not only to order it stopped prospectively, but to punish it as if it were a violation of a then-existing order. Such post-hoc exercises of regulatory power are wholly inconsistent with our system of justice. See generally Kaiser Aluminum & Chem. Corp. v. Bonjorno, 494 U.S. 827, 855, 110 S.Ct. 1570, 108 L.Ed.2d 842 (1990) (Scalia, J., concurring) (“The principle that the legal effect of conduct should ordinarily be assessed under the law that existed when the conduct took place has timeless and universal appeal.”).13
Given the constitutional significance of the duties Mr. Scofield'was obliged to carry out, we simply cannot agree that the district court was within its discretion to hinder his performance of those duties with an expansive, unprecedented, and after-the-fact “interpretation” of a rule of evidence. To the contrary, where an attorney seeks to perform his constitutionally mandated duty to effectively represent a criminal defendant, he must be free to interview defense witnesses and to discuss with them all appropriate matters, without being subjected to an overbroad sequestration order.
F.
While the district court concluded that Mr. Scofield violated the sequestration order, we are unable to find any such violation and conclude that there was none. As a result, the district court’s decision to exclude Alexander on the basis of a nonexistent violation was an error of law and, thus, an abuse of discretion. See Koon v. United States, 518 U.S. 81, 100, 116 S.Ct. 2035, 135 L.Ed.2d 392 (1996) (“A district court by definition abuses its discretion when it makes an error of law.”).
III.
Even if Mr. Scofield had violated a sequestration rule or order, we would still hold, in the context of this case, that the sanction imposed — exclusion of Alexander’s testimony — constituted reversible error.14 “Because exclusion of a defense witness impinges upon the right to present a defense, we are quite hesitant to endorse the use of such an extreme remedy.” United States v. Cropp, 127 F.3d 354, 363 (4th Cir.1997). Under these circumstances, we conclude that exclusion of Alexander’s testimony in its entirety was unduly severe.
At the outset, sanction analysis must encompass proportionality, and sanctions as extreme as witness exclusion must be proportional to the offense. Cf. Doyle v. Murray, 938 F.2d 33, 34 (4th Cir.1991) (“[Sjanctions [must] be fixed in proportion [322]*322to the severity of a party’s or lawyer’s misconduct[.]”). In this case, when the district court excluded Alexander’s testimony, it was aware, through Mr. Scofield’s representations, that Alexander had been exposed to Davis’s accusation that Alexander was involved in drug deals. However, the district court conducted no examination of Alexander to determine exactly what he had been told; neither did the court attempt to ascertain through Mr. Scofield what he had revealed to Alexander.
More importantly, the proffer of Alexander’s testimony covered at least six other Government witnesses and a number of other topics that were crucial to Rhynes’s defense. Mr. Scofield specifically represented that, “Your Honor, I do not believe, as I stand here and think about it, that I mentioned anybody else’s testimony other than D.S. Davis.” See supra note 4. Notwithstanding this representation, the district court determined that Alexander’s testimony relating to each of the other Government witnesses and other subjects was tainted by the same “coaching” as the testimony relating to D.S. Davis. Significantly, the court apparently made this determination based solely on Mr. Scofield’s proffer, see supra at 315, but on this record, the court’s finding of taint with respect to the other witnesses and other subjects is simply unsubstantiated. The exclusion of Alexander’s testimony in its entirety was thus unduly severe and disproportionate to the narrow scope of Mr. Scofield’s apparent violation.
We must also reject the Government’s contention that, in the circumstances of this case, the sanction was justified and proportional because the “search for truth” was thereby furthered. We agree, for example, that if Alexander had been permitted to testify and his testimony had contradicted Davis’s testimony, then one — either Alexander or Davis — was not, in all likelihood, testifying truthfully. However, the adversary system ordinarily can be trusted to separate the liars from the truthful. In this instance, the search for truth would have been better served by permitting the jury to determine who was telling the truth. By contrast, the district court’s exclusion of Alexander’s testimony left Davis’s testimony uncontradicted and may have led the jury to believe, and rely upon, untruthful testimony. Put simply, excluding Corwin Alexander as a witness did nothing to further the search for truth.
Further, the degree of fault or intent encompassed in the violation must be considered in ascertaining the propriety of any given sanction. See United States v. English, 92 F.3d 909, 913 (9th Cir.1996) (“One factor given considerable weight in determining what sanction, if any, is appropriate for the violation of a sequestration order is whether the side calling the witness deliberately violated the court’s order.”). Given the lack of legal support for the district court’s broad interpretation of its order, it is obvious that Mr. Scofield did not intend to violate the sequestration order. When questioned about his discussions with the witness Alexander in preparation of the defense, he was clear about his actions: “I specifically told [Alexander] about that testimony and told him I was going to ask him about that, Your Honor. And I don’t think that violates the sequestration order.” J.A. 1945M. An inadvertent misstep by Mr. Scofield was simply insufficient to justify the extreme sanction imposed on Rhynes.
We are also cognizant that although the alleged violation of the sequestration order was effected by Mr. Scofield, the sanction imposed inured to the defendant.15 There was, of course, no requirement that Rhynes be sanctioned for his lawyer’s conduct; indeed, a lawyer may be personally sanctioned for violations of court orders. If, however, a defendant is [323]*323being sanctioned for his lawyer’s conduct, courts should impose the least severe sanction justified under the circumstances. The district court had alternative sanctions at its disposal; we have endorsed at least two others: “sanction of the witness; [and] instructions to the jury that they may consider the violation toward the issue of credibility.” Cropp, 127 F.3d at 363. Further, there were many other possible corrective measures that could have been taken, including: limiting the scope of the witness’s testimony, see English, 92 F.3d at 913 (endorsing limitation of witness’s testimony following violation of sequestration order); permitting broad cross-examination into the alleged “coaching,” see United States v. Posada-Rios, 158 F.3d 832, 871-72 (5th Cir.1998); or any other sanction appropriate under the circumstances. There is no doubt that, under facts like these, the district court could have imposed a less severe sanction.
In short, even had Mr. Scofield’s contact with Alexander violated the sequestration oi'der, the district court abused its discretion in imposing the unduly severe sanction of excluding Alexander’s testimony in its entirety.
IV.
While harmless error analysis applies to the district court’s error, we are satisfied that the erroneous exclusion of Michael Rhynes’s sole corroborating witness, who would have sought to rebut much of the Government’s evidence against Michael Rhynes, was not harmless. We have here an error of constitutional magnitude — a deprivation of Michael Rhynes’s Sixth Amendment right to call witnesses on his behalf.16 For this constitutional error to be harmless, the Government is required to establish, to the satisfaction of this Court beyond a reasonable doubt, “that a rational jury would have found the defendant guilty absent the error.” Neder v. United States, 527 U.S. 1, 18, 119 S.Ct. 1827, 144 L.Ed.2d 35 (1999). When, as here, guilt “was genuinely contested, and there is evidence upon which a jury could have reached a contrary finding, the error is not harmless.” United States v. Brown, 202 F.3d 691, 701 (4th Cir.2000).
We need not dwell long on this question because it is clear that the erroneous exclusion of Alexander’s testimony was not harmless. Indeed, at the en banc argument in this case, Government counsel conceded as much, candidly acknowledging that, if Alexander’s testimony had been believed — -an assumption we must make in assessing harmless error, Michael Rhynes would have been found not guilty.
We agree with the Government. As Mr. Scofield’s unchallenged proffer of Alexander’s testimony made clear, Alexander would have sought to corroborate myriad details of Michael Rhynes’s own testimony, to discredit several of the Government’s witnesses, and to counter a number of the inferences the Government sought to draw from its evidence. See supra at 315. When he lost Alexander as a witness, Michael Rhynes lost the opportunity to independently challenge the Government on all of these issues. Faced with an error with such consequences, we are constrained to conclude that this error was not harmless.
V.
Pursuant to the foregoing, we vacate Michael Rhynes’s conviction and sentence and remand his case to the district court for a new trial.
VACATED AND REMANDED.