Revealing mistake: Near the beginning, during Tina's first dream, Tina gets cornered in front of one of the furnaces. She steps forward and Freddy jumps out from behind her and scares her. When she steps forward, Freddy (who is trying to lean down out of the shot) is seen stepping sideways to position himself to scare Tina.
Revealing mistake: When Nancy is running upstairs, and struggling in the goop holes, we see a shot of Freddy smash his arm through the small door window. If you play in slow-motion, you can see a large pre-made crack in the window to make it easier for him to smash, which WASN'T there when Nancy ran through...