It is possible that persons may be disqualified from serving as witnesses on grounds not expressly enumerated in codes of evidence or by general common law principles -- on unenumerated grounds such as age, illness, sensory defects, defects in intelligence, lack of understanding, inability to communicate, defects in memory, and inability to be truthful.

Crawford v. Washington, 541 U.S. 36, 68 (2004) ("Whatever else the term ['testimonial'] covers, it applies at a minimum to prior testimony at a preliminary hearing, before a grand jury, or at a former trial").If a statement was made in another judicial proceeding and the statement is offered against a defendant in a criminal trial, the statement - if offered to establish the statement is true - is "testimonial" and the statement is therefore subject to the right of confrontation guaranteed by the Sixth Amendment to the U.S. Constitution.

Loughan v. Firestone Tire & Rubber Co., 749 F.2d 1519, 1523-1524 (11th Cir. 1985) (action by an injured repairman for negligence and strict liability when a wheel assembly being repaired separated with explosive force; judgment in favor of the manufacturer; plaintiff argued that the district court...Evidence of habit may take the form of specific instances of conduct, opinion, or any other type of evidence otherwise permitted by the Federal Rules of Evidence.

Nowell v. Universal Electric Co., 792 F.2d 1310, 1314-1315 (5th Cir. 1986) (holding, in plaintiff wife's action against defendant electric company for wrongful death, that the trial court erroneously admitted the testimony [about an....The hearsay exception created by Federal Rule of Evidence 807 or a comparable State rule applies only if the the affected statement is evidence of a material fact.