Continuity mistake: The X-rays of Homer's intestine change sides between shots when he is at the vet.
Continuity mistake: When Lisa meets the other kids for the first time under the boardwalk, her tinted shades disappear in the very last shot.
Continuity mistake: When Marge is cleaning the dishes on the morning after Bart reveals Lisa as a nerd, Marge is seen in close-up with a white necklace. In next shot of Bart and Lisa, as well as the rest of the scene, Marge has a red necklace.
Factual error: Bart uses Milhouse's glasses as binoculars, but as Milhouse is near-sighted this wouldn't work.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.