Federal Due Process Clause limitations on admissibility of inculpatory evidence against criminal defendants.

United States v. Tanner, 61 F.3d 231, 237 (4th Cir. 1995) (upholding trial court's admission of the Puffenbarger testimony about accused's prior bad act of giving numerous illegal refills of Darvocet N 100 over a 5-year period because such evidence was relevant (1) "to show [accused's] modus operandi,"...Evidence of a criminal or wrongful act apart from the act that at issue at trial Is not barred by the prohibition against circumstantial use of character if intent, knowledge, mistake, or accident is a legally-material factual issue, the opposing party suggests the non-existence of knowledge or urges mistake or accident as a defense, and evidence of another crime, wrong, or act is offered to prove the knowledge or the absence of mistake or accident by a criminal defendant or other person.

Cf. Sparks v. Gilley Trucking Co., Inc., 992 F.2d 50, pin? (4th Cir. 1993) (negligence action based on negligent driving of defendant's employee; defense: negligent driving by plaintiff; prior convictions of plaintiff for speeding held inadmissible because negligence...Evidence of a criminal or wrongful act apart from the act that at issue at trial Is not barred by the prohibition against circumstantial use of character if intent, knowledge, mistake, or accident is a legally-material factual issue, the opposing party suggests the non-existence of knowledge or urges mistake or accident as a defense, and evidence of another crime, wrong, or act is offered to prove the knowledge or the absence of mistake or accident by a criminal defendant or other person.

Fed. R. Evid. 601 ("Every person is competent to be a witness except as otherwise provided in these rules.").Every person is a competent witness unless a statute or a rule of evidence provides otherwise.