A non-constitutional rule of evidence specifying when a person is or is not a competent witness cannot override a federal constitutional right to have a potential witness testify.

Fed. R. Evid. 803(23) ("Judgments as proof of matters of personal, family or general history, or boundaries, essential to the judgment, if the same would be provable by evidence of reputation.").There is a hearsay exception for judgments as proof of matters of personal, family or general history, or boundaries, essential to the judgment, if the same(?) would be provable by evidence of reputation.

United States v. Brown, 327 F.3d 867, 872 (9th Cir. 2003) (reversing conviction for wire fraud because district judge failed to give appropriate curative instructions after prosecutor, in closing argument, seeking to show defendant's criminal propensity, referred to evidence of defendant's other...A person's character, or propensity, is inadmissible to show that person's conduct on a specific occasion.

Zubulake v. UBS Warburg LLC, 382 F. Supp. 2d 536, 542-543 (S.D.N.Y. 2005) (holding, in an employment discrimination case, that evidence of employee's performance appraisal was inadmissible as habit evidence because her "insubordination at two securities firms [was] not of sufficient frequency...If a pattern of behavior of a natural person does not rise to the status of a habit, it may constitute a character trait and such non-habitual behavior may be barred by the prohibition against circumstantial character evidence.