Character mistake: During the car chase scene (with Wally and Dave in the cop car), at one point Wally hears a gun shooting at them and says to Dave, "Is that somebody shooting at us?", and Dave responds, "I didn't hear anything." Dave would realistically not have answered like that to Wally because when Wally asks that question, Dave (being deaf) was looking away from Wally and towards the front of the car, so he would not have been able to read Wally's lips to respond that way.
See No Evil, Hear No Evil (1989)
1 character mistake - chronological order
Directed by: Arthur Hiller
Starring: Kevin Spacey, Gene Wilder, Richard Pryor, Joan Severance
Other mistake: In the alley scene immediately following the jail escape Dave and Wally are trying to open a door, Dave steps away from Wally and while looking away says "There's an alley!" Wally hollers out "Dave!" and Dave whips back around and says "I'm right here, I'm right here!" He wouldn't have been able to have heard his name being shouted since he is deaf. (00:39:00)
Guy at convention: Doctor, which exercise would you find most beneficial to geriatric sexuality?
Wally: Oh boy. Well, some of my patients prefer walking. Some prefer bicycling. But for best result to guarantee satisfaction, most of them like fucking.