Revealing mistake: Towards the end of the movie, the principal puts on his hat which turns his hair green. But you can see the green paint has already been applied before he puts his hat on.
Revealing mistake: When Branch first takes Poppy into his bunker, and down into his storeroom, the lever that he uses isn't there, but when the shot goes to Poppy, and then returns to Branch, the lever is magically there.