Revealing mistake: Any time when the two girls are in hell's corridors and running around, you see these corridors as stone walls, yet whenever they back up against one of these walls, you can clearly see they are merely rubber sheeting overlayed on a framework as the rubber sheeting bends and contracts very clearly.
Revealing mistake: Several of the bodies hanging from the rafters in Channard's house that Julia had killed were obviously mannequins. Real dead bodies would hang heavily and slack from their bonds but at least two that I could make out weren't weighing on their ropes at all; they were posed, standing dummies.
Revealing mistake: When Kyle goes into the dead-body room, there is a shadow of a hanging dead person on the wall next to him. The shadow of the open door on the wall on the other side of him shows that the shadow of the body shouldn't have been there as the light would have to have been coming from the very far wall. When Kyle is looking around you can see that the only light source is the high windows on the left side of the room.
Revealing mistake: In the scene where Kirstie is in Hell searching for her father, she is running up and down Hell's corridor's, becomes winded and stops. As she does so, she slaps herself against a concrete corridor wall; when she does this, you can see that the "concrete" wall looks more like a rubber covering, as it moves with her.