How have courts interpreted the plain error doctrine in cases where a prosecutor made a statement that was inappropriate to the witnesses?

We are guided instead by cases, like this one, to which the plain error standard applied. In United States v. Berry, 627 F.2d 193, 198 (9th Cir.1980), the prosecutor argued the jury could trust each witness's story because the government had made a significant effort to keep the witnesses separate. We held that although the misconduct was significant, it was insufficient to invoke the plain error doctrine. Id. at 199. Likewise, in United States v. Gwaltney, 790 F.2d 1378, 1385-86 (9th Cir.1986), the prosecutor stated: "[T]hese men who testified here, ladies and gentlemen, are professionals and they will have to tell the truth." Again, we found that the statement may have been inappropriate, but did not constitute plain error undermining the fundamental fairness of the trial. Id. at 1386.