Continuity mistake: When Bart is first introduced to the Superfriends, Ham moves around the table a bit. First he is seen sitting next to the kid with dark, curly hair, and you can see a plate in between him and Bart's plate, indicating that someone else is sitting there. This someone turns out to be Lisa, who after she is introduced, switches seats with Ham, making Ham sit next to Bart.
Continuity mistake: At the end of the episode, when we see a wide shot of everyone going to join Ned on the hill (and singing the song) look to the far left. You will see Apu - and his skin is yellow.
Continuity mistake: The location of the comet in the sky changes direction at several points. Whilst it's apparent size and altitude will of course change, the direction it is coming from cannot. As an example, it is seen looming over the power plant towers, but when Ned is on the hill, it's no longer there.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.