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. (00:03:20)
Revealing mistake: During Tina's death scene before she is lifted into the air, you can see the wire attached to the right side of her waist. (00:17:25)
Revealing mistake: During Tina's death scene before she is lifted into the air, you can see the wire attached to the right side of her waist. (00:17:25)
Revealing mistake: When Nancy gets pulled under in the bathtub, you can plainly see it's the actress' stunt double. (00:33:20)
Revealing mistake: When Nancy sees Tina wriggling around in her body band and a centipede comes out of her mouth - you can see this is a doll that the centipede crawls out of. (00:38:35)
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)
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)
Revealing mistake: The day after Nancy comes out of her dream with Freddy's hat, she goes into the kitchen and drinks a big cup of coffee. Firstly she chugs a full cup of hot coffee. Ouch...how did she do that? Secondly she does it without swallowing once. Watch her throat. (00:49:40)
Revealing mistake: When Nancy runs downstairs and cracks the small door window open with the umbrella, you can see the window is precracked to make it easier to smash. (01:18:35)
Revealing mistake: Towards the end, when Nancy sets Freddy on fire, you can see the asbestos suit Freddy is wearing to prevent himself from getting burned. It makes his body blatantly fatter, and his mask has obviously been stretched to fit over the suit's head. (01:19:50)
Answer: The scene you are referring to is a dream sequence. In dreams everything is different and how they change throughout the movie is no mistake.
SAZOO1975