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
Directed by: Arthur Hiller
Starring: Kevin Spacey, Gene Wilder, Richard Pryor, Joan Severance
Continuity mistake: Wally and Dave exit the swamp after pushing the police car in. Look at Wally's legs; you can see a pair of Wellington boots underneath his trousers.
[Woman checks the back pockets of Wally's pants for the coin.]
Wally: I don't know what you're looking for, but it's a little to the right.