Revealing mistake: When Chris is trying to escape out of one of the first floor windows, the shots you see from outside are of a stunt double with really bad frizzy hair. (01:14:40)
Revealing mistake: When the snake leaps out of the cage, you can see a dodgy looking fake snake attached to a wire spring. (00:12:35)
Revealing mistake: When Debbie is in the shower and we see a shot of her through the shower curtain, you can see that she is wearing white underwear. (01:00:20)
Revealing mistake: During the opening sequence involving Harold and the clothesline, there is a foreign object on the lens of the camera - noticeable in the bottom left corner in several shots.
Revealing mistake: When Debbie is in her hammock, she looks up to see Andy who is dead. However you can see his hand moving. (01:02:25)
Revealing mistake: When Jason throws a knife after Chris you can see the wire attached to the knife. (01:14:25)
Revealing mistake: When Debbie is stabbed in the chest in the hammock, when the knife comes through you can see it's a fake chest made of rubber material.
Revealing mistake: Near the end of the movie, as they are about to walk out of the barn thinking everything is over and Jason has been hung on the hook from the loft, if you look between the cracks of the barn door, you can see Jason being lowered right before the doors open.
Revealing mistake: After Loco has been killed, it cuts to a shot of Fox, who is pinned up on the rafters with a pitchfork. She is clearly moving, however, despite trying hard to stay still. (00:40:40)
Revealing mistake: After Ali has smashed the front shield of the car it is obvious the window isn't made of real glass. It's made of plastic. (00:32:15)
Revealing mistake: When Harold is stabbed with the butchers knife, you can see the bulge and outline of protective padding under his shirt. (00:14:40)
Revealing mistake: Toward the end of the movie, Chris runs outside to find Rick after she finds a bloody dressing gown in the tub. When she goes by the tree, Loco falls down from the branches. When he falls you can see his closed mouth open a bit as if he was smiling, even though he is dead.
Revealing mistake: When Jason is squeezing Rick's head, just before the eyeball 'pops' out for the 3D effect, it's obviously a dummy's head, as the face is wider and slightly larger than that of the actor's.
Revealing mistake: Inside the barn, the webbing that the deer head is covered in is fake, as it 'pulls' like elastic when Shelly moves his along the skull. Real webbing would adhere to his bare skin.
Revealing mistake: When Jason is hitting Ali in the head with a club you see Ali move all three times he is being hit before the club is near him. (00:41:50)
Answer: The very ending of part 2 proves this, as Ginny appears again found by the police and ambulances. 1) if Jason really got her she would be dead, not alive, he wrapped around her. 2) Paul was dead, not alive and 3) as far as Jason's appearance she would not be able to stand it at first glance, so in her head she would put more human features on his face to be able to deal with it.