Continuity mistake: Right at the beginning, the first shot of Homer and Lisa shows the door open. After Homer says 'in the most wicked condition' and the shot changes the door is closed.
Revealing mistake: After Apu and Manjula go by in horse and cart everyone returns into Moe's. When they walk in they just disappear into a brown wall which is where the inside of Moe's should be seen.
Continuity mistake: Milhouse's hair is blonde in the school bus' mirror reflection.
Revealing mistake: When Homer says "Oh, baby we got him now" whilst arriving in the airport, you can see half of Doctor Hibbert's face in the background, but you can see he doesn't have any eyebrows or eyes.
Other mistake: When Nelson puts the intestine into the model's mouth, the size of the gut should not increase. It is the same amount of stuff going in a cycle, so where does all the stuff come from? (00:03:15)
Other mistake: The amount of gunk that come from Bart's model at the start is not proportionate to the size of the model. In other words, there couldn't be enough gunk to cover the entire inside of the bus.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.