I'm Goin' to Praiseland - S12-E19
Revealing mistake: In one of the scenes (I think it is when they first discover the alleged 'miracle') you can see people in the background walking out of the park. But they are not walking out of the park at all, they are walking into the wall.
Continuity mistake: In the last scene when Homer is about to attempt the loop trick, it is seen several times that the Mafia guys are not standing in a straight line. In all shots they are except the one where Homer fails, and then says "Ta da,". The Mafia are shown head to head in a straight line.
Continuity mistake: When Bart is trying to make the toilet water flow the other way, in one of the flushes the toilet begins to dispense water before Bart pulls the chain.
Continuity mistake: In the scene where Selma and Troy McLure get caught by the press outside the resturant, Selma and Troy swap places between shots.
Continuity mistake: At Moe's, when Homer takes hold of the telephone its color is red, but then changes to blue as he speaks to Marge. (00:02:30)
Continuity mistake: On the trip back from the gardens, Homer rear ends the car in front of him on the bridge which sends Maggie flying out of the car. In a long shot of the bridge whilst Maggie is in the air you can see the car has changed to an orange color instead of the usual light purple color. The two cars in front of them also change during shot. The one in front of the simpsons changes from a normal red mid sized car to a blue van, whilst the car in front of that one changes from being a green car to a blue one during the aerial shot of Maggie.
Diatribe of a Mad Housewife - S15-E10
Continuity mistake: Right at the beginning of this episode, Homer is wearing a watch on his left hand, and he is still wearing it in one shot when he's in the car, but when he reclines his chair it is gone. (00:00:30)
I, (Annoyed Grunt)-bot - S15-E9
Continuity mistake: The remote control for the robot in Bart's bedroom appears from nowhere. (00:08:30)
I, (Annoyed Grunt)-bot - S15-E9
Audio problem: After watching 'Robot Rumble', Bart tries to dissuade Homer from building a robot by listing his other "talents". The voice and Bart's mouth movements don't at all match. (00:06:20)
Homer's Barbershop Quartet - S5-E1
Audio problem: After Jasper has sung on the stage to Homer, Apu and Skinner, they simultaneously say "Next". But if you listen carefully you can't hear Homer's voice.
Burns' Heir - S5-E18
Continuity mistake: When Mr. Burns is sitting in front of the fireplace, he has his normal blue outfit when seen from behind, and a purple night gown when seen from The Front. (00:02:50)
Tree House of Horror II: A Simpsons Halloween - S3-E7
Audio problem: When Kang and Kodos land in Springfield and hit Mayor Quimby with a club, one of them says something but his mouth is not moving.
Like Father, Like Klown - S3-E6
Continuity mistake: In Krusty's story after he has his makeup washed off by a rabbi, just after his father speaks to him his makeup suddenly reappears. (00:09:40)
Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Deliberate mistake: In "Dial Z For Zombies", Homer is shooting zombies with the shotgun at the school. When he shoots the first two he just shoots them twice. Every other zombie he shoots requires him to slide the gun. It doesn't when he shoots the first couple of zombies. (00:19:50)
New Kids on the Blecch - S12-E14
Continuity mistake: When L.T. Smash switches the lever to Commission from De-Commission, after that, when the shot changes, it says Ship and Museum. (00:18:10)
Continuity mistake: When Homer bangs on the screen in the testing booth, the little sign on the top left of the glass that was there earlier is no longer there.
Secrets of a Successful Marriage - S5-E22
Continuity mistake: Marge puts down 2 glasses of juice and a baby bottle on the table when she is telling the kids about the marriage problems. but when she walks away in the next shot, the glasses and bottle are gone.
Springfield Connection - S6-E23
Revealing mistake: Just after when Marge is reading the book on the sofa she says 'chaos'. On the far left of the screen a large black 'scratch' appears then just slides off.
Tree House of Horror II: A Simpsons Halloween - S3-E7
Continuity mistake: In Dr. Marvin Monroe's office, in the first shot, we can see a garbage can beside his desk. It disappears for the rest of the scene. The desk also moves forward. (00:13:35)
Tree House of Horror II: A Simpsons Halloween - S3-E7
Continuity mistake: As the family looks out the window to see the limo, the light switch on the wall has vanished. (00:03:40)
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.