OPINION
EASTAUGH, Justice.I. INTRODUCTION
We reverse Conrad Worthy’s conviction for second-degree sexual assault and remand for retrial. Worthy should have been permitted to introduce extrinsic evidence challenging the truth of the complaining witness’s allegation that another man had raped her on a prior occasion, because her prior allegation was a crucial part of the state’s case against Worthy.
II. FACTS AND PROCEEDINGS
In July 1994 T.J.S. broke off her two-year romantic relationship with Worthy. But they remained in contact, and several weeks later T.J.S. agreed to dine with Worthy and another couple. On their way home from *773dinner, Worthy told T.J.S. that he wanted to stop at his office because he had been having computer problems. Inside the office, Worthy and T.J.S. began to argue. The argument became violent. The details are contested, but Worthy admitted that he physically assaulted T.J.S. T.J.S. claimed that Worthy also sexually assaulted her by digitally penetrating her.
Worthy eventually let T.J.S. go. She ran to a nearby store and called the police. The police took her to a hospital, where a medical examination revealed bruising and abrasion consistent with her allegations of physical assault and digital penetration.
The state indicted Worthy for first-degree sexual assault, among other charges. At trial, the jury acquitted Worthy of first-degree sexual assault, but found him guilty of the lesser offense of second-degree sexual assault.
Worthy challenges two evidentiary rulings made by the superior court. First, it excluded testimony tending to show that T.J.S. had previously falsely alleged that another man had sexually assaulted her on an earlier occasion. T.J.S. claimed that a co-worker named Chris had sexually assaulted her in Barrow in April 1994 following á work-reláted party. T.J.S. reported the alleged sexual assault to the Barrow police. The district attorney never filed charges against .Chris. At the time of the alleged Barrow sexual assault, T.J.S. and Worthy were dating and she discussed the incident with Worthy.' Then, during the encounter between Worthy and T.J.S., Worthy mentioned Chris. T.J.S. testified that Worthy punctuated his sexual assault on her by telling her to “think of [him] as Chris.”1
Before trial Worthy sought permission to call Chris to testify that T.J.S. had falsely accused him of rape and that T.J.S. had consented to have sex with him. The superi- or court refused to permit the testimony.
In its opening statement at trial, the state referred to the alleged Barrow rape and Worthy’s comments regarding Chris during the assault. T.J.S. also testified on direct examination about the'alleged Barrow rape and the trauma she claimed to have suffered as a result. The superior court limited cross-examination; the defense could discuss whether the sex was, in fact, consensual, and could only point out that no criminal charge resulted from the investigation.
Worthy argued before the court of appeals that the superior court’s ruling improperly restricted his right to litigate the Barrow rape. Worthy maintained that once the prosecutor asserted that a prior rape had really occurred, Worthy was entitled to introduce evidence to prove that no rape had occurred.
The court of appeals rejected Worthy’s argument, determining’that it wás essentially irrelevant whether a rape had actually occurred in Barrow; It noted that the Barrow episode was only relevant to give context to what Worthy told T.J.S. during the Anchorage incident.
Second, the superior court excluded expert testimony by Dr. Susan LaGrande, a psychologist, about the effects of childhood sexual assault upon an individual’s response to and perception of violent or upsetting nonsexual incidents. Worthy offered this expert testimony on the eighth day of trial. The superior court refused to allow this testimony because Worthy had not given timely notice of his intention to call this expert, and because Worthy failed to offer any foundational evidence to suggest that the expert’s theories applied to T.J.S.
The court of appeals did not consider the timeliness of Worthy’s notice because .it agreed that Worthy failed to establish the relevance of the expert’s testimony.
Worthy petitions for hearing from the decision of the court of appeals.
III. DISCUSSION
A. It Was Reversible Error to Reject Evidence that T.J.S. Had Previously Made a False Allegation of Sexual Assault Against Another Man.
The admissibility of evidence is largely within the trial court’s discretion and its *774rulings will not be overturned on appeal absent an abuse of its discretion.2
Worthy argues that the superior court erred in refusing to permit extrinsic evidence that T.J.S. had falsely accused Chris of sexual assault in Barrow. The state contends that the truth or falsity of the Barrow rape is irrelevant here; the only relevant evidence was that T.J.S. discussed the incident with Worthy and that Worthy told T.J.S. during the assault to “think of [him] as Chris.”
As a general rule, contradictory evidence may not be admitted if it relates to a collateral matter. If a matter is considered collateral, the testimony of the witness on direct or cross-examination stands — the examiner must take the witness’s answer.3 If the matter is not collateral, extrinsic evidence may be introduced disputing the witness’s testimony on direct or cross-examination.
There is no clear, bright-line demarcation between collateral and non-collateral matters. In Davenport v. State,4 we adopted relevancy as the dividing line; we stated that “facts which are relevant to the issues of the case [are not collateral].”5 McCormick on Evidence6 elaborated on this distinction: “the matter is non-collateral and extrinsic evidence consequently admissible if the matter is itself relevant to a fact of consequence on the historical merits of the case.”7
Is the truth or falsity of the Barrow rape relevant to this matter? As a general rule, the prior sexual conduct of an alleged sexual assault victim is not admissible.8 And the court of appeals has discussed an independent rule providing that an alleged victim’s prior false allegations of sexual assault are not admissible to discredit the victim’s current allegations unless the proponent of the evidence meets a threshold burden of establishing the falsity of the past reports.9 But we need not decide which of these two rules applies to Chris’s testimony. Because the state interjected the alleged Barrow rape into the case and made its occurrence a central part of the case against Worthy, the issue became independently relevant.
The state went much further than merely confining its evidence concerning the Barrow event to what T.J.S. had told Worthy about the incident and what Worthy had said to T.J.S. during the assault in his office.10 The state took it as a given that the Barrow incident Rad actually been a sexual assault. Worthy contends that the prosecutor relied on that assumption
to generate sympathy for T.J.S. as a victim of two rapes, to establish her as a truthful reporter of rapes, to show her mental condition as a rape victim, and to portray Mr. Worthy as a man so bad that he raped someone he knew was particularly vulnerable because she had just been raped by someone else.
We agree with Worthy’s characterization of the prosecutor’s approach.
The state’s trial references to the Barrow event had the effect of elevating that incident to a level of importance it would not normally have had.
We are guided here by our opinion in Davenport. Davenport was prosecuted for *775receiving and concealing stolen property.11 We approved of the prosecution’s request to introduce, for the purpose of impeaching Davenport’s testimony that he had never seen a certain gold cuff link before it was pawned, testimony linking the cuff link with defendant’s prior burglary conviction.12 We reached this conclusion because the proposed testimony “went to the essence of the critical transaction,” not to a mere collateral matter.13
McCormick on Evidence recognizes this category of non-collateral matters: “[A] part of the witness’s story may be attacked where as a matter of human experience, he could not be mistaken about that fact if- the thrust of his testimony on the historical merits was true.” 14
Chris’s proposed testimony is similar to the testimony allowed in Davenport and fits within the exception recognized by McCormick on Evidence. The prosecution chose to use the Barrow event to bolster T.J.S.’s credibility, generate sympathy for her, and tarnish Worthy’s image before the jury. . The prosecution made the truth of the Barrow rape a critical part of Worthy’s assault on T.J.S. For this reason, Worthy was entitled to litigate the truth or falsity of T.J.S.’s Barrow rape report. It was an abuse of discretion to exclude Chris’s testimony.
We will not reverse a conviction based on an evidentiary ruling if the error is harmless.15 Because the state made T.J.S.’s testimony regarding the alleged Barrow event an integral part of its case against Worthy, Chris’s testimony might have substantially affected the jury’s verdict. Exclusion of this evidence therefore requires reversal of Worthy’s conviction and remand for retrial. '■
Correctly noting that Worthy failed at trial to renew his pretrial request to call Chris, the dissenting opinion reasons that Worthy waived this argument.16 We decline to hold that Worthy did not preserve the issue.
Our reluctance is partly procedural. The state did not argue waiver in this court in its Brief of Respondent or at oral argument17 and instead argued the merits of the substantive issue. Had the state raised the waiver issue, Worthy might have persuasively explained in reply or in oral argument why we should not rely on waiver to avoid considering his ostensibly meritorious substantive argument. We could raise the waiver issue sua sponte at our discretion, but we are reluctant to do so where the state’s failure to raise the waiver issue was-itself a waiver that potentially prejudiced the opponent.
But our reluctance is mostly substantive. Under the circumstances of this case, we are not convinced that Worthy was required to do more than he did to preserve the issue. The defense .reasoned before trial that it was important to call Chris to show that the Barrow accusation was false. That reason never changed. Both before and after the state “opened the door” at trial, the truth of T.J.S.’s Barrow accusation was fundamental to the state’s úse of the incident. But the trial court had already ruled that there would be no attack on the truth of the Barrow accusation through testimony from Chris. Even after the state “opened the door” at trial, the trial court maintained its unwillingness. to address the truth of the Barrow accusation. Given the breadth of the trial court’s pretrial ruling, and its repeated assertions at trial that it was not going to allow the parties to try the Barrow incident, there is no reason to think the trial court would *776have reversed its pretrial ruling had Worthy-renewed his request.
Our refusal to raise a waiver issue sua sponte in this case should not be taken to indicate that we are generally willing to consider, absent plain error, appellate arguments not preserved below. Rather, the specific facts of this case and the countervailing preservation problem created by the state’s failure to raise the waiver issue lead us to conclude that we should not avoid the substantive issue on a theory that it was not preserved for appellate review.
B. Reversal Moots Any Emr in Excluding Defendant’s Expert Witness.
Worthy argues that the superior court abused its discretion by prohibiting his expert, Dr. LaGrande, from testifying to rebut the prosecution’s case-in-chief.18 The superior court based its exclusion decision in part on the untimeliness of Worthy’s expert witness disclosure.19 Because the exclusion of Chris’s testimony requires reversal and remand for retrial, it is not necessary to decide whether the court abused its discretion in excluding Dr. LaGrande’s testimony.
Assuming Worthy is retried, the superior court can revisit the foundation issue in context of the evidence before it at retrial. Worthy will also have an adequate opportunity to make a timely expert witness disclosure before retrial. Because Dr. LaGrande’s testimony will necessarily require reference to T.J.S.’s previous sexual conduct, AS 12.45.045(a) and Alaska Evidence Rule 404(a)(2) may affect the admissibility of Dr. LaGrande’s testimony. Alaska Evidence Rule 703 also potentially applies. And, as expert testimony, it must meet the requirements of Alaska Evidence Rule 702 discussed in State v. Coon.20
IV. CONCLUSION
We REVERSE Worthy’s conviction for second-degree sexual assault and REMAND for retrial.
BRYNER and CARPENETI, Justices, not participating.. On appeal, Worthy does not dispute T.J.S.’s testimony or characterization of his statements regarding Chris.
. See Hawley v. State, 614 P.2d 1349, 1361 (Alaska 1980).
. See Shane v. Rhines, 672 P.2d 895, 898 n. 2 (Alaska 1983) ("[E]vidence which is offered to contradict a collateral matter is inadmissible, whether or not the matter was brought out on direct.”).
. 519 P.2d 452 (Alaska 1974).
. Id. at 455.
. 1 John W. Strong, ed., McCormick on Evidence (5th ed.1999) [hereinafter McCormick on Evidence ].
. Id. § 49, at 203 (footnote omitted).
. See AS 12.45.045(a).
. See Covington v. State, 703 P.2d 436, 442 (Alaska App.1985), modified on other grounds on reh’g, 711 P.2d 1183 (Alaska App.1985). These threshold requirements have not been clearly expressed. See id.; Johnson v. State, 889 P.2d 1076, 1078-79 (Alaska App.1995).
. See Appendix.
. See Davenport, 519 P.2d at 453.
. See id. at 455.
. Id.
. McCormick on Evidence at 203.
. See Alaska R. Civ. P. 61 ("No error in the admission or the exclusion of evidence ... is ground for granting a new trial or setting aside a verdict ... unless refusal to take ’ such action appears to the court inconsistent with substantial justice.").
. Dissent at 777.
. Nor did the state argue waiver in its appellee brief before the court of appeals or in its response to Worthy's petition for hearing.
. See Shepard v. State, 847 P.2d 75, 79 (Alaska App.1993) (vesting trial court with broad discretion to regulate admission of expert testimony and reviewing decision to exclude expert testimony for abuse of discretion).
. Because Alaska Rule of Criminal Procedure 16(c)(4), which requires the defense to provide notice of experts no later than thirty days prior to trial, did not become effective until after Worthy's trial, the question of what constituted timely notice was committed to the trial court’s discretion. See Supreme Court Order No. 1191. Here, the superior court’s pretrial order required the defense to disclose its expert witnesses two weeks before trial. Compliance with a pretrial order is required unless justice demands otherwise. See Alaska R.Crim. P. 22(b) ("[W]hen entered [the pretrial order] shall control the subsequent course of the proceedings, unless modified at the trial to prevent manifest injustice.”). Worthy did not comply with the pretrial order as entered.
.974 P.2d 386 (Alaska 1999).