In US v. William Harris, No. 05-2016 (3d Cir. 12/29/06), the 3rd Circuit held as a matter of first impression in the Circuit that "asking one witness whether another is lying is inappropriate." But since there was no objection in this case to the prosecutor's questions of defendant regarding whether the police witnesses were lying, the issue was reviewed for plain error. Since the Circuit had not yet ruled until this case, the error was not "plain." Nearly all other circuits have ruled that such questions are improper and may constitute reversible error if a timely objection is made.
These sorts of cross-examinantion questions are particularly common whenever a defendant testifies. Thus, in light of Harris, it is now especially important to object anytime the prosecutor asks the defendant (or any other defense witness) if the police witnesses must be lying.