The habit of a natural person is admissible to show the person's conduct on a particular occasion.

Fed. R. Evid. 404(b)(2) ("On request by a defendant in a criminal case, the prosecutor must: (A) provide reasonable notice of the general nature of any such evidence that the....The prosecution's use of other acts or crimes for nonpropensity purposes in a criminal case is permissible under Federal Rule of Evidence 404(b) only if the prosecution, if accused so requests, provides reasonable notice in advance of trial, or during trial if the court excuses pretrial notice on good cause shown, of the general nature of any such other bad act or other crime evidence it intends to introduce at trial.

Washington v. Texas, 388 U.S. 14, pin? (1967) (state rule barring accomplice from testifying for accused accomplice held unconstitutional).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.

United States v. Green, 305 F.3d 422, 428-429 (6th Cir. 2002) (upholding trial court's ruling which allowed the government's questions to witnesses on cross-examination about their knowledge on a specific airport incident involving the accused after the witnesses testified on direct examination as to...Although a character witness who testifies to the reputation of another person or gives an opinion of the character of another person cannot testify to specific instances of that person's conduct to bolster the character witness's testimony, an opposing party may question the character witness about specific instances of the person's conduct to show that the character witness' testimony about the character of the person was not well-founded.