When Marge unplugs the fan, we see the blades of the fan exposed with no grille. Yet when Maggie holds the fan up to the dog, there is magically a grille.
When Willy is knocked unconscious by the falling letters, 3 Ns fall sideways next to his head, making it appear as if he is sleeping/snoring, yet there are only 2 Ns in "Springfield Elementary School."