Plot hole: When the cop shows Pam the newspaper clippings they include what look to be fresh photos of the real Jason from the events of Part IV, yet no photographer is ever seen taking pictures of Jason alive in the previous films and most likely wouldn't live long enough to publish the photo. (01:25:00)
Revealing mistake: When Ethel gets a cleaver in her face, blood spatters everywhere. Yet the cleaver blade is still clean. (00:56:35)
Plot hole: When the couple go into the woods to have sex, after they finish, the guy goes to a pond to clean up, which is not very far from where he left his girlfriend. A couple minutes later, "Jason" is about to kill her, and she screams very loudly. Despite this he doesn't hear her.
Revealing mistake: When Jason is raising the machete above Tommy in the woods, it's obvious that his eyeholes are covered with black cloth. This also happens at the end of the movie, where Tommy hallucinates about Jason in the hospital. (00:04:45)