Revealing mistake: In the fight scene at the pier, after Freddy and Jason get thrown into the air by the carriage that smacks into them, they land on the pier in sort of slow motion. Watch Jason's left knee as he lands, you can see a kneepad.
Revealing mistake: When Freddy and Jason are fighting towards the end of the movie, Freddy cuts Jason's fingers off so he drops his machete. But when Freddy catches it you can easily see that it isn't a real one (it's made of rubber, or something like that). (01:20:40)
Revealing mistake: When Jason is slicing kids at the party, you can see the padding under their shirts
Revealing mistake: When Linderman dashes at Jason and impales him with an American flag, Will is dragging Lori (who is supposed to be unconscious). He's dragging Lori by her arms... but when he suddenly reaches behind his back with his right arm, her right arm stays up in the air. (01:13:50)
Revealing mistake: When the drug "Hypnocil" is researched on the Internet, an article about the drug appears. If you pause the shot and read the article, you can see that it has nothing to do about Hypnocil. The article is about drug testing of students who engage in extracurricular activities. (00:55:10)
Revealing mistake: As soon as the drunk kid sets Jason on fire there's a few quick shots of Jason's face but there's a perfect hole right through his head where his eye is supposed to be and you can see the night sky through it. Some people say it's a reflection off the goggles to protect the actor from the flames. Either way it's a mistake. (00:40:45)
Revealing mistake: In the final fight scene where Freddy and Jason are outdoors, Freddy gets his leg caught in a rope and is hung upside down. This causes his pant leg to rise revealing an unburned limb, which obviously was not covered with make up. (01:19:25)
Revealing mistake: In the scene were Freddy and Jason are fighting at the end, when Freddy puts the machete in Jason you can easily see a rip in the shirt made for the machete to go in.
Revealing mistake: In the cabin fight scene while Jason is trying to free his machete from the table, Kia picks up a wrench and starts hitting him. When she hits him across the face, you can see the wrench bend in her hand.
Revealing mistake: Obvious rubber material when Lori tears off Freddy's ear. (00:56:40)
Revealing mistake: At the end, Jason tears off Freddy's arm but when they go flying into the lake you can see Freddy has both arms intact.
Revealing mistake: When Jason kills the kid by throwing his flaming machete through his body, it is obvious that the guy has a mouth full of fake blood in the slow motion shot, right before Jason impales him with the torch. (00:41:15)
Revealing mistake: When Jason sends Kia's body flying into the tree during the fight scene at Crystal Lake, you can see the dummy's arm go flying off of the dummy, in the upward direction, in the shot after the body makes contact with the tree.
Revealing mistake: Notice when Jason is stabbing Trey. Jason isn't even close to Trey at all. He's just stabbing the mattress.