Other mistake: After his wife has a party at their house, Thorton Melon (Dangerfield) decides to have a swim in his pool. The camera shows Melon as he is about to dive into the pool from the edge. The camera cuts away and a much "thinner" Melon is seen doing a flip into the water. Watch carefully as the "thinner" man makes his dive. You can clearly see his hairpiece fly off the top of his head.
Continuity mistake: When the snooty English economics professor first spots Thornton and his son in his class, we can see two empty seats directly in front of them. However, when the camera moves to the back of the room, the two chairs are filled. This happens a couple of times during the scene as the camera angle changes.
Continuity mistake: During the fight scene in the bar, after the pep-rally, Lou (Burt Young) is seen pressing a guy over his head. The guy is wearing a denim vest, with a red sweater on underneath. After Lou throws the guy's head into the jukebox, the guy who is sticking out of the jukebox is wearing a full denim jacket. The red sweater and vest disappear.