Plot hole: Freddy is killed at the end because he sees his own reflection in a mirror, which causes the souls in his body to revolt and kill him. But this weakness to mirrors and reflections doesn't fit in any other film of the series. He repeatedly appears in mirrors in the other films, and in the climax of "Dream Warriors," he even appears in a hallway of mirrors that are facing each other, where he would have seen his own reflection multiple times. No matter how you slice it, his death in this movie doesn't add up in the overall context of the series.
Continuity mistake: When Freddy is reaching out to take Sheila's glasses off, you can see that the palm and index finger of his hand aren't scarred at all from what was supposed to be an all-consuming fire. (00:43:55)
Continuity mistake: When Dan is wheeled into the operating room, we hear a respirator machine going in the background. Yet he's fully awake and not hooked up to any breathing equipment when he's being brought in. A machine would not be going like that unless the patient was fully under and intubated.
Revealing mistake: After the truck has its collision with nothing, the chain yanking it backwards is visible. (01:09:00)