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.
Continuity mistake: The door to Mrs. Krabappel's classroom has a window, but when Bart fakes being sick then walks out of the class (we see the desks as the door closes), the door's window has vanished, and the doorknob has switched sides as well. Additionally, earlier, when Bart deliberately falls off his chair in class, note that one of the bars on the back of his chair has gone missing. (00:07:35)
Continuity mistake: As the workmen begin to leave site after hearing that Homer is unable to pay them, a couple of tools are dropped on the ground; however, in the following shot, facing back towards the cabinet where Homer is hiding, the tools have disappeared.
Continuity mistake: When Bart fails the test despite having actually studied, we see the two red "X" marks in the center row of the multiple choice questions, but after Mrs. Krabappel passes Bart because he applied practical knowledge, when Homer hangs the test up on the fridge those two "X" marks in the center have vanished.