Revealing mistake: After Freddy stretches the blackboard vertically, it switches to a closeup shot of him peering out from the side of the board. If you look closely at his glove, you'll see clear plastic caps on the tips of the knives. You'll also see them when he scrapes the board with his pinky and the final time he runs his hand across the board.
Revealing mistake: When Freddy controls Spencer to the top of the stairs, Spencer stands right at the edge of the top stair, and leans forwards. As he does so, if you look at his shadow on the wall you can blatantly see the shadow of the harness and thick wire attached to him, keeping him from falling.