Revealing mistake: During the part where J.J. goes down to the boiler room to play her guitar, Jason sneaks up behind her. She screams and runs down more stairs, and then right before Jason kills her, as he lifts the guitar to hit her, in the lower right hand of the screen you can see the blood coming out of the guitar before it hits her.
Continuity mistake: Rennie's visions of Jason as a boy change their appearance constantly throughout the film. At some points he is bald and deformed, much like how he was in the original film. At other times he appears as a normal, non-deformed boy with a full head of brown hair. At some moments he even appears as a mix of both (with a deformed face, but a full head of hair). This inconsistency is never explained or even acknowledged.