Continuity mistake: When Bart and Lisa are trying to win back Homer's affection they are sitting on the top of the stairs and push the skateboard with the present on it down the stairs. In the next shot it rolls into Homer's legs yet it would be impossible as he was standing in the main living room which is to the right of the stairs. Pushing the present down the stairs it would go straight into the front door.
Continuity mistake: In "Desperately Xeeking Xena", as Lucy Lawless grabs the lips of the Collector, you can see a big "C" on the back of his cape. After she has thrown him across the room and he gets up, the "C" is gone. It reappears later in the fight.
Continuity mistake: When Bart is standing on the bench, Jimbo and Milhouse are standing in the front row, on the right side of the screen. But when there's the closeup (a second later), there are children shown in front of Jimbo and Milhouse and the girl with the purple hair is gone.
Continuity mistake: In "Wiz Kids", Bart stabs Montymort in his shin which is open (i.e. not covered by what he's wearing). Only a little blood pours out. Then Monty raises his shin into shot and there is not only a puncture in the leg of his clothing, but the wound is covered in blood. When he falls, and Slithers eats him moments later, there is no damage at all.
Continuity mistake: When the hamster Nibbles has rolled back to Skinner, it takes the lid of the ball off, and the lid lands on the ground. When the hamster is rolling away from Skinner it doesn't put the lid back on, it just leaves it on the ground, but in the next shot the lid is suddenly gone.