Exclusion of evidence in criminal trials due to a violation of a criminal defendant's constitutional rights

Fed. R. Evid. 703 ("The facts or data in the particular case upon which an expert bases an opinion or inference may be those perceived by or made known to the expert at or before the hearing.").If the testimony of an expert witness satisfies the special requirements for the admissibility of expert evidence, the expert witness may give an opinion based on evidence and facts about which the expert witness has no personal knowledge: an expert witness need not be a "percipient witness."

Buckley v. Mukasey, 538 F.3d 306, 319 (4th Cir. 2008) (vacating judgment and remanding for further proceedings in a retaliation claim in which the trial court excluded Segar litigation evidence, on ground that plaintiff "intended to utilize the Segar litigation evidence to demonstrate retaliatory animus,...Another act, wrong, or crime is admissible to show a motive for some alleged conduct on a particular occasion.

Anderson v. Franklin County, 192 F.3d 1125, 1129 (8th Cir. 1999) (deaf and mute witness held to be incompetent under the circumstances).It is permissible for a trial court to disqualify a person as a witness -- to rule that a person is an incompetent witness -- because the person has a serious physical defect or infirmity that prevents the person from properly functioning as a witness.