Continuity mistake: When John and Maggie are at the old school, there is a teacher talking to a class which isn't really there. The teacher stands by the left side of the blackboard, which has large crosses drawn on it, in rows. It cuts to a closeup and the crosses are now smaller and more scattered.
Continuity mistake: John opens the window to find out the house is falling, and a large object goes flying through the window next to the bed, smashing out all the glass and all the wooden frames. Seconds later all the wooden frames are now repaired, however all the glass is still missing.
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.
Other mistake: Why hasn't there been a mention of Freddy having a child in previous films? Couldn't be because they covered it up, as soon as Freddy was discovered to be the Springwood slasher the media would have been all over him and his family.
Plot hole: The pipe bomb that Maggie uses to kill Freddy is the same one that was found in Spencer's room earlier in the film. It's already been established that Freddy was erasing the people he's been killing from existence, so that pipe bomb wouldn't be there to use on Freddy.