Saddlesore Galactica - S11-E13
Continuity mistake: During the BTO concert, whenever the camera focuses on the crowd, the Simpson family is seen at The Front of the crowd. When the camera focuses on the stage, the Simpsons are at the very back of the crowd.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.