Subject to limitations, a witness may be impeached -- the credibility of a witness may be attacked -- by evidence of untruthfulness (lack of veracity), bias and interest, inability to understand and mental disorder, inability to communicate, inability to perceive, or prior inconsistent statements.

United States v. Williams, 989 F.2d 1061, 1068 (9th Cir. 1993) (upholding trial court's admission of witnesses' testimony [recounting statements made by defendant Williams and deceased adopted son, Mark Reitler concerning....Federal Rule of Evidence of Evidence 804(b)(3) or a comparable State rule creates a hearsay exception for a statement which was at the time of its making so far contrary to the declarant's pecuniary or proprietary interest, or so far tended to subject the declarant to civil or criminal liability, or to render invalid a claim by the declarant against another, that a reasonable person in the declarant's position would not have made the statement unless believing it to be true. A statement tending to expose the declarant to criminal liability and offered to exculpate the accused is not admissible unless corroborating circumstances clearly indicate the trustworthiness of the statement.

Fed. R. Evid. 1003 ("A duplicate is admissible to the same extent as an original unless (1) a genuine question is raised as to the authenticity of the original or (2) in the circumstances it would be unfair to admit the duplicate in lieu of the original.").A duplicate of an original is admissible to the same extent as the original if (1) there is no genuine question about the authenticity of the original, and (2) admission of the duplicate in lieu of the original is not unfair.

United States v. Blount, 364 F.3d 173, 181 (4th Cir. 2004) (upholding trial court's admission into evidence of the (1) testimony of the district court's deputy clerk relating to an incident in the Clerk's office before trial, and (2) mother's plea agreement where the testimony was offered "to prove that...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.