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.
Revealing mistake: When everyone is in Ned's shelter, the woman in the top left hand corner with brown hair and brown skin briefly has 2 pupils in each eye.
Other mistake: While Kent Brockman says "This reporter smells another local Emmy" after he gets back into his van, his face is drawn incorrectly.
Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: In one scene in Bart's treehouse, Wendell's skin is yellow, rather than its usual white color.
Continuity mistake: When Roger Meyers throws his nameplate at the writer the second time, the animation is repeated from when he threw it at him earlier. It's more obvious because his mouth doesn't match what he's saying and the background is orange instead of purple like the rest of the scene.
Brother From the Same Planet - S4-E14
Revealing mistake: The shot of Homer saying "Marge, when kids say bad these days, they mean good. And to shake your booty means to wiggle one's butt" is recycled from the opening scene from Life on the Fast Lane. Notice the animation is of a much lower standard than the rest of the episode.
Marge vs. the Monorail - S4-E12
Continuity mistake: Doctor Hibbert was among the passengers on the monorail on the first ride, but he is seen with a pair of conjoined twins in his office soon after.
Revealing mistake: When Homer points at the donut guy, his right shirt cuff is colored yellow. (00:12:50)
Revealing mistake: Krusty's nose flashes white for a split second during his song for the winner of the Little Miss Springfield pageant. (00:15:06)
Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Audio problem: In "Clown Without Pity" when Homer yells "The doll is trying to kill me, and the toaster's been laughing at me!" his mouth doesn't move. (00:06:45)
Continuity mistake: When Chief Wiggum pulls Homer over for speeding, all the clown decorations on his car have disappeared.
Revealing mistake: In one angle when the Flanders are having dinner, Ned's eyeglasses have no temples.
Continuity mistake: When Ned is saying grace, the plates on the table contain no food, but when Homer comes through the window there's food on the plates.
Krusty Gets Kancelled - S4-E22
Continuity mistake: When Bart and Lisa are helping Bette Midler pick up trash on the highway, Bette picks up an empty can which says Buzz Cola on it. Just as she yells, "You pigs!" the label on the can switches to Duff Beer.
Revealing mistake: When Marge is yelling at Homer at the dinner table after Lisa goes to her room, the outline of Marge's mouth over her neck when her lips are moving shows the neck line in her lips. (00:18:50)
Revealing mistake: When Barney says "How long will the story take?" after Bart offers to tell the mob his story, the upper half of Barney's face starting from his nose is a darker shade of yellow than his lower half. His shirt also changes colour. (00:01:40)
Revealing mistake: In the beginning, when the angry mob is chasing after Homer and Bart, there are two Dr Marvin Monroes. One of them is behind Mrs Krabappel and the other is next to Grampa Simpson. (00:01:40)
Continuity mistake: At the end when Nelson's goons are holding on to Nelson, they both have brown skin. But in the next shot, one of them goes back to having yellow skin.
Continuity mistake: When Homer is rolling around on the grass outside the house with Bart, Lisa, Santa's Little Helper and Snowball II after the hair formula works, Snowball II is drastically larger than usual in the overhead shot.
Revealing mistake: While Patty tells Selma "Look who's talking" after Homer debuts his new haircut, both of their chests up to their necklaces are the same color as their dresses. Selma's dress also suddenly has sleeves on it.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.