Can a prosecution witness falsely deny the existence of a leniency agreement?

We agree with the district court that where a prosecution witness falsely denies the existence of a leniency agreement, there is no need to prove a deliberate design to suborn or conceal perjury on the part of the prosecution. Giglio v. United States, 405 U.S. 150, 154, 92 S.Ct. 763, 31 L.Ed.2d 104.

On the other hand, when the prosecution simply fails to turn over a particular item of evidence, without involving perjury, the issue of materiality for constitutional purposes "must reflect our overriding concern with the justice of the finding of guilt," United States v. Agurs, 427 U.S. 96, 112, 96 S.Ct. 2392, 2401, 49 L.Ed.2d 342 and "if the verdict is already of questionable validity, additional evidence of relatively minor importance might be sufficient to create a reasonable doubt." 427 U.S. at 113, 96 S.Ct. at 2402.