Visible crew/equipment: When Freddy falls over the banister onto the stairs near the end, notice the crash mat he lands on. (01:18:55)
Visible crew/equipment: When Rod is being strangled by the sheet in the jail cell, watch as it goes around his neck and you can see the wire in the sheet that pulls it. (00:42:20)
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)
Continuity mistake: In Nancy's final showdown dream, she goes down to the cellar, then goes to open the door which leads to the boiler room. When she opens the door it's hinged on her right hand side, yet when the shot changes the door is now hinged on her left hand side. (01:13:25)
Continuity mistake: When Krueger smashes his arms out the front door window, large pieces of glass are still hanging in the window, but in the next shot, as he pulls Nancy's mother through, all the glass has disappeared. (01:25:10)
Continuity mistake: Keep watching when Glen sneaks through Nancy's bedroom window for the lighting in the bedroom. When the exterior shots are shown the light is always on but it is sometimes off inside.
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)
Plot hole: Following Glen's death, Nancy tells her father to come across the street in 20 minutes to catch Freddy. She then goes to her house, checks and sets her alarm clock, and proceeds to rig the entire house with booby traps, including filling a light bulb with the contents of several shotgun shells, and has a heart-to-heart talk with her inebriated mom. When she gets back to her room to go to sleep, she checks her clock again. The time on the clock has advanced only ten minutes, not nearly long enough for everything she did.
Visible crew/equipment: When Nancy "jumps" from the boiler room to her front garden you can see her land in a pile of bushes. You don't even have to look closely to see the big black air mattress she lands on. It actually bounces above the fake bushes and becomes almost completely visible. (01:17:00)
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.
Continuity mistake: When Freddy jumps out from behind the tree in Tina's dream, his glove is on his left hand then it cuts away and back it's now back on his right hand. (00:15:50)
Continuity mistake: When Nancy is at her bedroom window, she says "OK Krueger, we play in your court" and she is holding her alarm clock. She then pulls down the blind, then when it cuts to inside, Nancy is walking over to her bed but isn't holding her alarm clock. If you look, the clock is on her bedside table, but she doesn't put it there. (01:11:25)
Continuity mistake: After Nancy wakes up from her nightmare in the dream clinic, her mother discovers her arm is scratched. The tape that was on Nancy's chin has gone, but returns in the next shot, only to disappear in the next. (00:48:50)
Visible crew/equipment: When Nancy's mother is pulled through the door window, a tape marker can be seen on the blue door, above the door handle. (01:25:15)
Visible crew/equipment: When Glen walks through Tina's backyard to investigate the noise, right after the bushes rustle when he says "Kitty kitty," the camera progresses forward, and the leg of a crew member is visible for a second at the bottom right hand corner of the screen. (00:08:50)
Continuity mistake: After Nancy pulls her phone out in frustration, she wraps the phone cord around it, and places it on her bed nice and neatly. But in the next shot of the phone mysteriously ringing, the phone is in a different position and the phone cord is wrapped around the phone differently. (01:03:50)
Visible crew/equipment: This is during the part where the girl dies in the bedroom with her boyfriend (where she gets dragged up along the roof). Just before, when you see the marks made by Freddy's glove, and her hands come up to show her pain, she clearly grabs a chunk of the thin latex pad (easier in slo-mo but not vital). (00:16:35)
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 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)
Continuity mistake: In the boiler room Nancy finds Glenn's bloody headphones. As she raises them up from the ground, between shots, the way she holds them changes. (01:16:30)
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