Revealing mistake: When the Hotel manager walks in the recently vacated room, we see the piano collapse. If you look closely, you can tell that the front two legs were just pulled back under the piano causing it to collapse.
Bachelor Party (1984)
1 revealing mistake - chronological order
Directed by: Neal Israel
Starring: Tom Hanks, Tawny Kitaen, Adrian Zmed, George Grizzard
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.
Mrs. Thompson: I had a wiener right in my hand.
Chosen answer: It's showing he's sexually adventurous, as he's using a cooking appliance as a sex toy.