In the case of Sherrod v. Berry (856 F.2d 802 (7th
Cir. 1988)), Sherrod was killed by a policeman named Berry, and Sherrod's
family sued for wrongful death. At trial, Berry testified that he thought
Sherrod had a gun, but no gun was found on Sherrod. The Trial Court admitted
the evidence about there not being a gun, and the jury found for Sherrod. The
Appellate Court reversed.
The Appellate Court found that evidence about whether
Sherrod was actually armed, (information Berry did not have at the time)
was improper, irrelevant, and prejudicial to the
determination of whether Berry acted reasonably 'under the circumstances'.
Basically, since Berry reasonably thought that Sherrod
had a gun, evidence that later proved he didn't was not relevant
to determining if Berry acted appropriately or not.
This case came to the opposite conclusion as Knapp v.
State (168 Ind. 153, 79 N.E. 1076 (1907)). In that case it was argued that
the underlying fact was relevant to determining if the belief was reasonable.
You could argue that if Sherrod did not have a gun, then Berry's claim that he saw a gun was less believable.