Continuity mistake: At the end in the hospital, Maggie's bow is red for a second, then it changes to blue.
Continuity mistake: All through out the episode, Bart draws his pictures with only one pencil, but they all end up in color when he's done.
Continuity mistake: When Homer falls into the pool with green paint, most (but not all) of him is covered with paint. In the next close up shot, he's completely covered in paint.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.