In Tanner v. U.S. (1987), 483 U.S. 107, 107 S. Ct. 2739, 97 L.Ed.2d 90, the court noted that the nearly universal common law rule in the United States flatly prohibits "the admission of juror testimony to impeach a jury verdict." 483 U.S. at 117. An exception is "recognized only in situations where the jury was allegedly influenced by an 'extraneous influence.'" Id. However, the Supreme Court also noted in Tanner that defendants have a right to "a tribunal both impartial and mentally competent to afford a hearing." Id. at 127. In this context, the Supreme Court observed that lower federal courts had previously employed a common law exception allowing "postverdict inquiry of juror incompetence in cases of 'substantial if not wholly conclusive evidence of incompetency.'" Id. at 125, citing U.S. v. Dioguardi (C.A.2 1974), 492 F.2d 70, 80. In Tanner , the Supreme Court refused to decide whether Fed. R. Evid. 606(b) could be interpreted as retaining this common law exception. Instead, the Supreme Court decided that the submitted evidence fell far short of satisfying the standard for juror incompetence. Id.