Revealing mistake: When all the kids are in the diner just before Chunk knocks over the water jug, Mikey starts to bang on the ground with a bar. When Brand stops him, Mikey calls him by his real name Josh. This is verified on the DVD by Sean Astin himself, in the commentary. (00:40:40)
Revealing mistake: When the cook's body is lowered to the kitchen floor, you can see her move her outstretched arm closer to her head.
Revealing mistake: When Jesse is cleaning out his room, he puts a tape into his tape player, and presses a button down on the top of it. But halfway though the scene of him packing his things away, you can clearly see the button is no longer pressed down, yet the tape still plays. (00:18:10 - 00:19:00)
Revealing mistake: In the closeup of Tina's dead body, her chest moves. (00:44:00)
Revealing mistake: When Mombi's headless body wakes up and begins to walk towards Dorothy, you can see a black, blurry haze around her neck in two shots, where an effect was used to remove it.
Revealing mistake: There are several instances where you can see that the bandages used to wrap up the girls are pre-assembled. When the first girl is "waxed" you can see there are no joins between the bandages, they are all in one piece - almost like a body stocking. You can also see this later with Elizabeth, and it becomes more significant because you see the bandages being wrapped around her head as normal, and then when Watson pulls them off, they come away in one piece like a balaclava, which of course shouldn't happen.