Revealing mistake: When Nancy is running up the stairs in her house to escape Freddy for the first time her foot hits the steps and go through the steps into some goop. Look at the steps above that step and you can see the pre-cut holes where the next sets of goop will be. (00:39:10)
Visible crew/equipment: When Nancy is in the dream clinic the doctor attempts to give her an injection. As she pushes the doctor away, the bedsheet falls slightly and Freddy's hat can be seen for a couple of seconds. In the next shot, the sheet is above her waist again at which time she reveals that she pulled the hat from her dream.
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... (00:39:15)
Factual error: When Glen and Nancy are talking on the bridge, you can see huge palm trees in the background. Though this was filmed in California (hence visible palm trees at the cemetery), the fictional Springwood is supposed to be in Ohio - they show the location of Springwood on a map at the start of 'Freddy's Dead, the Final Nightmare', and there's no other conflicting in-universe location given within the series. So it's established as Ohio, but there are no palm trees in Ohio. Other California "hints" add to the mistake, such as CA license plates, etc. (00:01:00 - 00:43:22)
Continuity mistake: In one scene Nancy's mother is pouring herself a drink, then Nancy enters the room and she switches off the TV. Then Nancy and her mother start talking about going to school, and behind Nancy's mother on the worktop, a white alcohol bottle can be seen. But after their conversation when Nancy leaves, the bottle of alcohol is gone. (00:19:40 - 00:20:30)