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)
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)
Strong Arms of the Ma - S14-E9
Audio problem: The speech does not match the lip movements from when Bart says, "dad, do you plan to get in shape?" until Lisa says, "Uh guys, mom just left the basement." (00:12:17)
Simpsoncalifragilisticexpiala(Annoyed Grunt)cious - S8-E13
Deliberate mistake: During the final musical number, just after Marge sings "I'm going bald from stress," the family sings "But we're happy just the way we are," but Homer's mouth doesn't move, because the producers forgot to record Dan Castellaneta's lines.
The Fabulous Faker Boy - S24-E20
Other mistake: When Zhenya says "Boris" while showing Bart the scales, she presses the A key instead of the B key. (00:10:00)
Other mistake: When the Amish are building the tree house, the top of the tree is missing. It's just sky. In the next shot the top of the tree is back.
Revealing mistake: During the intro to some earlier episodes where Bart leaves the school, the scene pans down and the edge of the animation paper can be seen in a couple spots at the very bottom of the frame. It is blue in color. This error persisted through many seasons.
Tree House of Horror VIII - S9-E4
Continuity mistake: Homer is dancing naked in the church and has his pants on the pew. There is then a wide shot and his pants are gone.
Burns Verkaufen der Kraftwerk - S3-E11
Other mistake: When Burns is writing on the back of the big cheque, the writing suddenly appears in one block. (00:10:05)
Burns Verkaufen der Kraftwerk - S3-E11
Continuity mistake: After the Germans have bought the plant, Homer takes a bite of his doughnut. The bite changes size between shots when Lenny and Karl ask him why he can't be fired. (00:09:10)
Burns Verkaufen der Kraftwerk - S3-E11
Continuity mistake: When the two Germans are looking at Homer in Moe's, the one's hand is on his paperwork. Camera cuts and it's half off the paperwork now. (00:05:20)
Burns Verkaufen der Kraftwerk - S3-E11
Continuity mistake: Homer pours his bottle of beer into his glass creating foam that is raised above the lip of the glass. Camera cuts and the foam has gone. (00:05:20)
Burns Verkaufen der Kraftwerk - S3-E11
Continuity mistake: Moe opens the special Duff and foam appears down the bottle that disappears instantly. (00:05:10)
Audio problem: When Moe says "I'll talk to you tomorrow" to Homer, his mouth doesn't move. (00:15:15)
Continuity mistake: When Moe gets up on stage with Aerosmith you can see the big "Flaming Moe's" sign behind them. However in the close up the sign isn't visible. (00:13:30)
Audio problem: When Krusty arrives at the bar, Bernard says "Hey Krusty" but his mouth doesn't move. (00:11:00)
Continuity mistake: When Martin tells Bart about the answers being worthless, Bart is stood by the grass or the kerb depending on the shot. (00:04:00)
Other mistake: When watching TV at the start Homer's beer can just says D instead off Duff. (00:02:15)
Continuity mistake: When Bart gets off the bus there is a fire hydrant behind him. When we see the bus drive off the hydrant has gone. (00:07:10)
Continuity mistake: When Bart gets on the bus the day of the test, Milhouse is sitting at The Front of the bus. When we see Sherri and Terri looking at Bart, Milhouse has gone. (00:06:30)
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.