Continuity mistake: When the school bus drops off Bart and Milhouse at the Simpson house, Bart puts his backpack on frontwards and starts krumping. When he walks through the front door, he is still wearing his backpack on his front. It then cuts to inside the house, and we still see him coming through the door, but the backpack is gone.
Continuity mistake: When the men from Festival Cruise Lines arrive at the Simpson house, they park in the left-hand lane (closest to the house) with the car facing oncoming traffic, seen both when they first pull up and, more, in the background while informing Milhouse about his parents. But in the overhead shot of the house, the car is parked the other way around, following the direction of traffic.
Continuity mistake: In the second segment of "Treehouse of Horror XXI," as Roger is telling them about what happened to him, as he finishes the story of Marge and Homer saving him, in his recount Marge is wearing a shirt and bikini bottoms (the shirt is covering the bikini bra). But when they saved her, she was just wearing a bikini.
Continuity mistake: When the students have to practice square dancing in the gym, Lisa spots a male student and there is a red circle next to her on the floor where she is standing. When the teacher tells Lisa she has to dance with groundskeeper Willie, the circle has disappeared.
Continuity mistake: In a full view of the checkout, when the cashier requires a price check on Gramma's Shortcut Store Bought Bundt Cake, you can see beside Marge there is a stand and there is a man with a beard and glasses behind it. When we see Marge in the next shot, the man is now standing behind her, and there is also a couple who weren't there previously.