Other mistake: Near the beginning of the episode when Selma and Sideshow Bob arrive at the Simpsons' house, Bart and Lisa scream and hide behind Marge. Selma says "What the - ?" but the DVD subtitles say that Marge says it. It is shown "MARGE: What the - ?". (00:02:30)
Other mistake: How can Sideshow Bob make the "I H8 BART" licence plate if there are no number keys on the keyboard? (00:04:45)
Other mistake: When Sideshow Bob leaves Selma to watch MacGyver, the room opposite is 350. Later when he is being taken away by the police there is a room 44. It's unlikely these two numbers would be on the same floor.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.