Other mistake: If Freddy ripped his scalp off, it would be his skull, not his bare brain. It's not like he pulled both of them off at once. Freddy's got a skull, as we saw when Tina took his face off in the first film, so why is the top of it missing in this one?
Suggested correction: This scene takes place within Jesse's nightmare, which is Freddy's domain, so Freddy can manipulate any detail he wants, in this case, exposing his brain without cutting open his skull.