Continuity mistake: When Homer is eating donuts while everyone is doing push-ups, his donut stays the same size every time he bites it.
Other mistake: When Moe is rolling his eyes after saying it was the first time he poured beer into the water supply, Herman's face is drawn incorrectly.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.