Continuity mistake: After Lisa wakes from her daydream, there's a boy behind her with blonde hair and a light purple shirt. After Ms. Hoover says "Then come back and finish the test", the boy's hair turns blue, and after it zooms in on Lisa's face, there's a boy with brown hair and a blue shirt behind her.
Continuity mistake: While in the cafeteria, Principal Skinner asks Milhouse to lower his eyebrows. Behind him, there is a bulletin board that changes from brown to purple.
Continuity mistake: While Lisa is standing in front of the table with her friends in the cafeteria, we see that the trays appear during the first shot, then after the camera zooms in on Lisa's table, the trays disappear. Then during the third shot, Lisa calls Alex over to her table. Look at the table again: the trays have reappeared. (00:06:00)
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.