Continuity mistake: When Homer is driving home in a bad mood (just before he gets stopped by Chief Wiggum), you can see that there is a pattern on the side of the car. In a next shot when Wiggum is walking alongside the car there is no pattern.
Continuity mistake: Just before Homer (dressed as Krusty) is picked up by Officer Wiggum, his car is brightly painted and has wobbly clown wheels. When he pulls over it's back to the ordinary dented purple car he usually drives.
Continuity mistake: In the last scene when Homer is about to attempt the loop trick, it is seen several times that the Mafia guys are not standing in a straight line. In all shots they are except the one where Homer fails, and then says "Ta da,". The Mafia are shown head to head in a straight line.
Continuity mistake: When Homer arrives at the Van Houten house for Milhouse's party, Janey is among those at the entrance and she appears with yellow skin twice.
Continuity mistake: When Chief Wiggum pulls Homer over for speeding, all the clown decorations on his car have disappeared.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.