Visible crew/equipment: In the playground scene, when Dylan jumps from the top of the rocket, it's an adult stunt double.
Continuity mistake: Julie punches a nurse and she passes out on the floor. She closes the door and locks it. Next scene shows the same nurse outside the door trying to get in.
Continuity mistake: When Julie is being dragged by Freddy on the ceiling you see traces of blood where she has been dragged. But there is also blood in front of her where she hasn't been dragged yet. (01:18:05)
Visible crew/equipment: When Heather is leaving to go to her TV show, she and Julie walk out Heather's house. As they walk out and talk, you can see a boom mic dip down into view for a while. (Seen on DVD version) (00:13:25)
Continuity mistake: At the hospital when Nancy says "Dylan, you have to fight it", it cuts during the sentence. First Dylan is looking at Heather, but when it cuts he is looking down. (00:54:25)
Continuity mistake: After Porter has destroyed the glove controller, the other guy is by the table. The controller isn't there, but when it cuts, he is suddenly by the controller throwing it to the floor. (00:03:30)
Continuity mistake: While the junior nurse is whispering to Dr. Heffner, "I don't think she ever lets him get a nights sleep." You can see that both of her hands are behind her back, but in the next shot her left hand is up by her neck. (01:14:55)
Continuity mistake: Towards the end of the film Nancy takes a big kitchen knife to fight Freddy with. When she is falling into Freddy's lair from her son's bed, she clearly has nothing in her hands. However, as she emerges from the water she has fallen in, the knife appears in her left hand. (01:29:10)
Continuity mistake: After Heather discovers the long scars on Chase's body, she turns to be sick. In one shot, she has her hand by her mouth, then when the shot changes her hands are down by her side. (00:33:45)
Factual error: When Dylan wakes up in the hospital, under a plastic tent, we hear the iconic "flat-lining" sound of a heart monitor. Dylan then removes the clamp from his finger and the sound stops. The clamp is an oximeter, a device that measures blood oxygen concentration. The wires going under his shirt are the heart monitor. The sound should not have stopped. (01:12:42)
Chosen answer: Mostly just very subtle hints. In the beginning when she arrives right around the same time that Heather gets a call from the psycho-stalker, suggesting a possible connection. And the fact she's alone with Dylan many times during the film, as she could be subtly manipulating him to act out.