Revealing mistake: When Shelia is dying, Alice is trying to help her, but she is 'thrown' to the floor. Then there is another shot of Shelia, and you clearly see a pulse in her neck, although she's meant to be dead. (00:44:50)
Revealing mistake: During Rick's nightmare, we can see the fake reflection trick in action when Kristen is taunting him in the mirror. When she turns around, the actor playing her reflection turns out of sync, and brings the wrong hand up to her mouth. (00:53:13)
Revealing mistake: After her brother's death, there is a scene in which Alice starts practicing with her brother's nun-chuks. They never show the real actress doing the tricks; instead, they cut to an angle from behind her and show a double who has darker, shorter hair than the real actress and is a bit bigger. (00:56:45)
Revealing mistake: When Alice is being blown towards the cinema screen, she grabs hold of one of the cinema chairs. You can clearly see she is actually holding a handle which is on top of the chair. This is blatantly visible when she lets go of it and flies off. (01:01:35)
Revealing mistake: After the truck has its collision with nothing, the chain yanking it backwards is visible. (01:09:00)
Revealing mistake: When Kristen is crawling on the ceiling in Freddy's house after falling through the sand, her hair is still plastered against her head, neck, and shoulders, not hanging down making it obvious that the crew shot the scene upside down.
Answer: As Kristin was being burned to death in the boiler, she passed her ability to pull other people into her dreams to Alice. This enabled Freddy to get new victims.