Revealing mistake: When Lisa is clapping erasers, you can see the alphabet above the board, and part of the A is cut off.
Revealing mistake: After Bart gets promoted to Hall Monitor, when he tells the girl at the fountain to keep the line moving, the kid behind her drinks from the fountain but the water does not go into his mouth. (00:13:40)
Revealing mistake: On the original DVD version, when Bart says to Lisa "I didn't want you to wreck your life" the screen is cut off on the right side slightly. (00:20:25)
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.