Whenever there is an exterior view of the Matthews' residence, you see a window on the left of the house, but you can see no coat rack. Inside, there is one. This applies to every episode.
During detention with Mr. Feeny, on his desk, there are two pencils in his cup container. They move positions throughout the detention scene, yet, no one ever touches them.
Mr. Feeny's milk carton changes positions between each take when he's eating lunch and when Cory comes over to see whether Mr. Feeny loves or hates him.
Mr. Feeny tells the boy playing Romeo to stab himself, but in the play Romeo dies by drinking poison and Juliet stabs herself.