Other mistake: In the beginning of the movie, when the mechanic is working on the car on the lift, he says he has to change the oil in it, the chassis drops to the ground, there is no engine attached to the chassis or body. In addition, he is changing the oil at the rear axle.
Deliberate mistake: The car Rudy, the mechanic, is working on is a 1971-72 Plymouth (the one where the chassis falls down around him). Plymouth's of that era were unibodies, which means that the body and the chassis were all one piece and not separate, so there's no way a chassis could have fallen down like that from that particular car, since it didn't have a separate chassis.
Visible crew/equipment: A short scene opens with an alternate couple in an opposing hotel suite to the one the bachelor party is in full swing at. The guy relaxes the woman by telling her he is just going to open the window for some fresh air, and as he walks to that window and is about to open it, the boom mike can be seen. This appears on the Australian VHS release of this movie. (01:16:30)