Other mistake: During the shot when Marge asks Homer who 'Hank Jones' is, you can see that the couch curves down on the left side of the screen, therefore putting her on the other side of the couch. (00:07:40)
Other mistake: When Bart and Homer do a test drive on their race car, it is daytime, when supposed to be night-time. (00:11:50)
Other mistake: When we first see Nelson's Roadkill 2000, Nelson is moving backwards, though we see his legs as moving forward. (00:13:25)
Other mistake: At the finish line, the crowd are missing all their appearance features such as faces, hair etc. (00:13:50)
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.