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. 404(b) (Evidence of other crimes, wrongs, or acts "may... be admissible for other purposes, such as proof of motive, opportunity, intent, preparation, plan,....A person's other crimes, wrongs, or acts are not barred by the character rule either if they are not offered to show that person's propensity or if they are not offered to show that that person engaged in conduct on a particular occasion because of his or her propensity to engage in that sort of conduct.

Cf. United States v. Lawrence, 405 F.3d 888, 902-903 (10th Cir. 2005) (upholding trial court's refusal to admit the Dr. Mitchell's statements to the FBI because "the statements did not have any guarantee of truthworthiness, as....A statement is admissible under the hearsay exception created by Federal Rule of Evidence 807 or a comparable State rule only if circumstantial evidence shows that the affected statement is as trustworthy as the statements that are made admissible by the hearsay exceptions created by Federal Rules of Evidence 803 and 804 or by comparable State rules.

Cf. United States v. Zedner, 401 F.3d 36, 49-50 (2d Cir. 2005) (upholding trial court's admission into evidence of rebuttal witnneses' testimony about accused's previous commission of fraud, where accused claimed lack of criminal intent, and evidence of fraud was relevant to prove accused's...Evidence of another crime, wrong, or act is not barred by the prohibition against circumstantial use of character if such other crime, wrong, or act is offered to prove the existence of a plan or scheme.