In the last confrontation Nancy has with Freddy, you see her pushing him down the stairs. Yet if you look closely you can see the mattress that the "Stunt" Freddy falls on.
Nancy sets up the sledgehammer booby trap and runs down the stairs. Freddy opens the door in pursuit of her, setting off the trap, and the impact of the sledgehammer causes him to stumble and fall over the railing. This is where you see Freddy land comfortably on a very obvious stunt mattress.
If you look carefully when Tina (Amanda Wyss) is first slashed by Freddy's claws on her torso, you can see as the blood starts to pour out, and Amanda pulls her hands up to stop it, her arm catches on the prosthetic and pulls it up slightly at the bottom left corner of the screen.