Revealing mistake: When Lisa is explaining to the curator of the museum why she didn't tell the crowd of what she learned about Jebediah Springfield, if you look in the background none of the people on the far sidewalk are moving, almost frozen in place with some of them with their arms in the air yet everyone and everything in the parade keeps moving right along.
22 Short Films About Springfield - S7-E21
Revealing mistake: A black line briefly appears on Apu's forehead while he's tossing the tofu dog around.
Team Homer - S7-E12
Revealing mistake: When the children are running down the hallway, the animation keeps repeating. Duplicates of the same children are running several times.
Scenes From a Class Struggle in Springfield - S7-E14
Revealing mistake: When Lisa is first shown riding the horse, the horse's eye is colored solid red.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.