Marge vs. the Monorail - S4-E12
Plot hole: When Homer is trying to stop the Monorail, he grabs the big M from the side of the train and uses it as an anchor. However, when the M cuts through 'Springfield's Oldest Tree', there is no visible monorail track or supporting pole in the background.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.