Continuity mistake: Whilst Chalmers notices the Honda symbol missing from his car, there are four holes in the corners of the leftover symbol. There is a screw in one of the holes indicating the symbol would have holes in its corners to hold it down. Later when Skinner finds the Honda symbol in Nelson's locker you can see there are no holes in the symbol.
Other mistake: After Ugolin (one of the French men looking after Bart) has taken Bart to the Chateau Maison, Cesar and Ugolin speak. (they aren't visible in this shot) Ugolin says "Escape is impossible", but the subtitles say that Cesar says it and vice versa for when Cesar actually speaks.
Continuity mistake: As Marge arrives home and sees the bullet holes in the door, the shot changes to a POV shot from behind the door and you can see there are some changes in the holes. The first change is there are three bullet holes on the right hand side which are all the same distance from each other, when she enters the middle hole has moved closer to the bottom one. The other change is the large hole in the middle of the door. On the bottom right hand side its shape goes from being a little irregular to a more round shape.
Continuity mistake: In "Terror at 5 1/2 Feet", when Milhouse decides that he is going to sit next to Uter, Uter all of sudden moves to the other side of the seat between shots.
Deliberate mistake: When Homer pushes the trampoline off the cliff, there is no rock below the falling trampoline. However, in the shot at the bottom a tall rock is visible which was drawn there on purpose for the trampoline to bounce off of, despite not being visible in the first shot.
Continuity mistake: In the beginning after Maggie hit Homer on the head, it shows him laying on the couch with Bart & Lisa on the left. The next shot it shows the TV, in the next shot it shows Homer on the left sitting and Bart & Lisa on the right. It's impossible for Homer to get up and switch sides with Bart And Lisa in less than 5 seconds.
Continuity mistake: At the end just before the Simpsons say "Happy Halloween everybody" they are about to bite Lisa (and make her a vampire). Maggie has no dummy (or in America, pacifier) in her mouth, but it suddenly appears when she closes her mouth.