Every person is a competent witness unless a statute or a rule of evidence provides otherwise.

United States v. Nazemian, 948 F.2d 522, pin? (9th Cir. 1992) ("we have held repeatedly under the plain error doctrine that a contention that the government failed to comply with the confrontation clause involves substantial rights and thus must be reviewed even in the absence...Despite the absence of an objection, it is plain error for a trial court to admit evidence offered against a criminal defendant or other disadvantaged party when the evidence is clearly, plainly, or obviously inadmissible and the admission of the evidence is likely to prejudice the defendant or other party against whom the evidence is offered.

Pennsylvania v. Ritchie, 480 U.S. 39, 58 (1987) (accused charged with sexual offenses against his daughter; held, accused had constitutional right "to have the CYS [Children and Youth Services] file [which was confidential under state law] reviewed by the trial court to determine whether it contains...Although sparse United States Supreme Court dicta and precedent suggest that nonconstitutional privilege rules may have to yield to a criminal defendant's federal constitutional rights, except for the specific and rare circumstances in which the Court has said or suggested that must happen, it is unknown when such privilege rules must yield to a criminal defendant's constitutional rights such as the right to due process and the right to present a defense.

Cf. United States v. Naiden, 424 F.3d 718, 722-723 (11th Cir. 2005) (upholding trial court's exclusion of defendant's statement [that he did not believe his online acquaintance to be fourteen years old] as a state of mind....To fall within the hearsay exception provided by Federal Rule of Evidence 803(3) or a comparable State rule for person's present internal states, it must be shown that declarant had no chance to reflect, fabricate, or misrepresent his thoughts.