Continuity mistake: When Trish's car stops, the licence plate goes from dirty to clean. (00:28:10)
Continuity mistake: When Trish and Tommy push the cupboard in front of the door, the figurines and items on the shelves move/fall off/reappear between consecutive wideshots and closeups. (01:15:10)
Continuity mistake: Nurse Morgan's name tag is on the opposite side to the pocket with the pen in it, on her white nurse dress. Yet when Jason stabs Nurse Morgan, her name tag is right next to the pocket. (00:12:00)
Continuity mistake: When Jason kills Doug in the shower, there are two red lines on his hockey mask, under the eyes. However later, when Jason bursts through the window and grabs Tommy, the lines are gone. (01:05:10 - 01:14:45)
Chosen answer: He's not possessed. Dealing with Jason made him go crazy. In part 5, we find out he was in a mental hospital for a while, then gets sent to a home for troubled kids.