Homer's Paternity Coot - S17-E10
Continuity mistake: When Dr. Hibbert is about to reveal the results of the paternity test, Mason Fairbanks is wearing a blue blazer and black pants. But, when Dr. Hibbert points at him and says "This guy", he has a light blue sweater and blue pants.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.