Continuity mistake: During the lamb vision at dinner Lisa is holding her knife and fork. In the very next shot they're on the table.
Continuity mistake: When Sadgasm is making their first appearance, Homer says, "We are Sadgasm," and there are a few students walking around, but when he says, "And this song is called 'Politically INcorrect'," there is a big crowd in front of them with no time to form one.
Miracle on Evergreen Terrace - S9-E10
Continuity mistake: When Bart pulls the melted tree out of the house, a shovel appears out of thin air so he can shovel snow onto it.
Lisa's Rival - S6-E2
Continuity mistake: When Nelson does his "Grapes of Wrath" demonstration, it splatters all over Skinner's jacket. When he walks over to the rest of the dioramas his jacket is clean again.
Homer's Barbershop Quartet - S5-E1
Audio problem: When grandpa is listening to the radio, he says something like 'oh.', but his lips are not moving.
Revealing mistake: When Lisa is daydreaming about her failed test, the president of Harvard talks to her. When he says, "Nasty business that zero", his right white sleeve turns the same color as the glass above the door.
Homer Badman - S6-E9
Continuity mistake: When Homer mixes the Pop Rox with the soda he leaves the Pop Rox packaging over the can. After he throws the can and we see it in the air the Pop Rox packaging has disappeared.
Lisa on Ice - S6-E8
Other mistake: As Homer is signing Bart's academic alert form just before Marge starts talking to Lisa you can read the writing below the "Academic Alert" title, but the writing is completely incoherent including sentences such as "IMAEGTSIAD TSNHOJI NAMREB". Update; some of the "words" or parts of them are people's names, such as David, Stuart and Allison, but backwards. (00:03:50)
Large Marge - S14-E4
Continuity mistake: After Homer paints "El Homo" on the wall, he paints over it so that none of the letters are visible. In the next shot a couple of the letters are visible, and are only struck out with one line.
The City of New York vs. Homer Simpson - S9-E1
Continuity mistake: At the very start of the episode, Moe is wearing a watch. But in the next shot when all the guys come in, and all following shots, the watch is gone.
The Great Louse Detective - S14-E6
Continuity mistake: Frank Grimes' gravestone appears to look different from "Homer's Enemy" and "Alone Again, Natura Diddly."
Tree House of Horror X - S11-E4
Continuity mistake: When the flashback of running over Ned happens his jumper is ripped and sleeveless because he has just been a wolf. Earlier though when the Simpsons find him, his jumper is not ripped.
Continuity mistake: Throughout the whole of this episode, the clock on Mr. Burns' desk keeps appearing and disappearing.
Radio Bart - S3-E13
Continuity mistake: When Rod and Todd talks to 'God' on the radio, the antenna keeps changing position for the entire scene. (00:08:50 - 00:09:40)
Tree House of Horror VII - S8-E1
Continuity mistake: In 'The Thing and I' Dr. Hibbert is about to explain Bart's irregular birth. In the closeup before the story, Dr. Hibbert has a stethoscope around his neck. This wasn't there in previous shots. (00:03:50)
Large Marge - S14-E4
Continuity mistake: In some shots, the incorrectly-colored lobsters have their claws taped shut, and in others they don't. It's completely random.
Tree House of Horror VIII - S9-E4
Continuity mistake: When Homer pulls up to the stoplight, the number of cars in front of him changes. When he first stops, there are 3. When he gets out of the car, there is only 1.
Continuity mistake: When Milhouse is spying with his binoculars with an camouflage outfit on, the colours of some patches change in some other shots. E.g The bottom left corner (from behind) changes from dark green to light green.
Revealing mistake: When Lisa and Hugh go by Taxi to the Simpsons house, you can see that they pass the same buildings all the time, especially "Coffee shop".
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.