Continuity mistake: Towards the end of the movie Dave takes Walley's jacket and uses it to help them slide down a cable. Before they slid down, Dave tells Walley to hold on, and Walley asks if they're going to be above the ground. Dave tells him no, and off they go. However, Dave is deaf, and reads lips, and Walley is standing behind him when he asks the question. So how did Dave read his lips if he wasn't looking at him?
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.
Revealing mistake: Dave and Wally are checking into a hotel. Dave is looking down and signing in on the paper while the clerk says to them, "we have a room with 2 queens." Dave (still looking down, never at her lips) looks up and says "Well get them fellas out of there." Dave is deaf and would not have made that comment, seeing that he never looked at her so couldn't know what she said.