Continuity mistake: When Maggie is opening her presents, the biggest present is pink. Then after Patty and Selma walk away, the biggest present is blue.
Continuity mistake: Moe's back pockets, the pockets on his bum, change position/disappear between shots - before the Italian-American-Mexican stand-off.
Continuity mistake: In the scene where Maggie swallow' Moe's penny, she first takes out her pacifier to eat it, but in the next shot it's in her mouth again and in the next, she's putting it back in her mouth.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.