Revealing mistake: In the opening sequence's parody of the series where Bart becomes fat and comes out from school on his skateboard, he theoretically should go down the stairs, instead his silhouette suddenly climbs up on the wall next to him, revealing a mistake in the post-production's animation. (00:06:05)
The Mook, the Chef, the Wife, and Her Homer - S18-E1
Revealing mistake: When Fat Tony says "The Calabresis. My archenemies in waste management." while driving the carpool, a huge line briefly appears around his cheek and mouth.
Smoke on the Daughter - S19-E15
Revealing mistake: When Lisa turns on the light while dressed in her Angelica Button costume, her mouth disappears for a single frame.
Revealing mistake: When Alberto says "If you had completed your seduction of me, I never would have met Sylvia," his shirt collar briefly turns into the same color as his blazer.
Mypods and Boomsticks - S20-E7
Revealing mistake: When Bashir leaves the Simpson house, he doesn't have any eyebrows. (00:12:40)
Revealing mistake: At the craft store when Homer and Bart go in, the door opens before Homer pulls the handle. (00:14:00)
Revealing mistake: While Lou is talking to Chief Wiggum about having his belt stuck to a street sign, Lou's mouth disappears for a single frame.
Revealing mistake: When Homer is riding Barney like a bull at Moe's, there are duplicates of Sam and Larry (the barflies) in the same shot.
Politically Inept, with Homer Simpson - S23-E10
Revealing mistake: Right before Homer is patted down at the airport, his facial hair briefly flickers yellow.
Politically Inept, with Homer Simpson - S23-E10
Revealing mistake: While Homer catches his underwear with his tongue after the Simpsons' clothes burst out of the suitcases, Homer's tongue briefly passes through his underwear.
Ned 'N' Edna's Blend - S23-E21
Revealing mistake: When Marge is talking to Helen Lovejoy about Canapés at the party, Marge's mouth disappears for a single frame.
Revealing mistake: When Reverend Lovejoy arrives at church in the hot tub, there are two Carls right next to each other in the crowd.
Revealing mistake: At the circus as the clowns appear you can see a composition error on Homer's body. Homer's body is indexed to be as a background scene for several seconds where you can see the man's head fully who is sitting in front of Homer but you can just see that the man's head is in front of Marge's body. You will also see the clown's shoes walk over Homer's body before they update Homer's body correctly. (00:05:19)
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.