What is the effect of prosecutor's overkill on the testimony of a witness in a criminal case?

. . . are prosecutorial overkill. They inevitably give jurors the impression that the prosecutor is carefully monitoring the testimony of the cooperating witness to make sure that the latter is not stretching the facts something the prosecutor usually is quite unable to do; . . . If proper objection had been made to the summation, the judge should have sustained it; if matters had gone too far to make a striking of the remarks an effective cure, the judge should have instructed that the promise in the cooperation agreement adds little to the truth-telling obligation imposed by the oath; that the prosecutor often has no way of knowing whether the witness is telling the truth or not; that the books are not filled with perjury indictments of Government witnesses who have gone beyond the facts; and that an acquittal would not mean that as a matter of course the Government would seek such an indictment or even fail to make its promised recommendation of leniency. If prosecutors know that such instructions will be given, they will hardly be tempted to the excesses committed here.

United States v. Arroyo-Angulo, 580 F.2d 1137, 1150 (2nd Cir. 1978) (concurring opinion).

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