Revealing mistake: When the scientists inject the mouse with poison, they put it in a jar. The, after they place it down you can see someone's hand swap the jar for one containing a dead mouse. You can even hear it. (01:17:58)
Revealing mistake: In the library scene when Richie sees Pennywise the clown, all the balloons filled with blood are popping on people who aren't supposed to be able to see them, but one lands on a lady and you can see her jump as the balloon pops.
Revealing mistake: In the opening scene where the zombie kills Johnny. When they fall against the tombstone you can tell it is a dummy.
Revealing mistake: When the lawn mower the heroes send off as a diversion crashes you can see a wire pulling it. (01:01:30)
Revealing mistake: When Melanie Griffith comes home from the jail, as she opens the door you can see there's no wall, it's just the door.
Revealing mistake: In "Cat from Hell" the cat crawls back out of Halston's mouth after the clock strikes twelve at Drogan's return. As it exits, the mouth and face collapses and is quite obviously made of rubber. (00:54:45)
Revealing mistake: In the hotel meeting scene, the Grand High Witch tells the witches they can take their wigs off. When we see the newly bald witches, it is particularly obvious that only the foreground witches are ladies with bald caps - the rest of the witches are bald men. (00:30:10 - 00:31:00)
Revealing mistake: During the party scene, when Jacob starts hallucinating, you can see the stick that one of the fake birds is attached to.
Revealing mistake: When Danny falls down from the ceiling, watch closely and you can see the security wire attached to his leg. (00:34:40)
Revealing mistake: The Gremlin who throws the beaker full of acid has his arm operated by a stick, which is visible as he chucks the beaker. (01:08:05)
Revealing mistake: When Strakk is firing the giant nail gun thing at Darkman during the climax, you can see that the gun firing does not always match the squibs going off on the beams behind Darkman.
Revealing mistake: When Joanne is kneeling by Phil's body, she cries "Oh God no." and then, just before it cuts, Phil's right hand moves, even though he is dead. (00:45:50)
Revealing mistake: When Nelson first undergoes the experiment, the doctors use a small mask to put him to sleep. When he is supposedly waking up, Rachel asks Randy to help her with oxygen. He hands her the same small mask they used the first time ie. anesthetic. Just for a second she has a look on her face which says "this is the wrong mask" and she tries to hide it with her hand. For the whole rest of the film, the other doctors use a bigger mask when giving oxygen.
Revealing mistake: Ken is killing Diane by stabbing her with the knife. You can see that Ken is only stabbing the floor next to her.
Revealing mistake: When young Norman Bates is spying through the peephole on his mother, who has brought home a man to the motel, we see him in the shower. Although the curtain is translucent, we can see he is wearing a black speedo.
Revealing mistake: When the boys use the glasses to start the fire, there is smoke coming from under the wood they are lighting. This wouldn't happen. It would come only from the focused point of light made by the glasses.
Revealing mistake: At the start of the film, where West and Cain are re-animating dead soldiers, one of them tries to kill West. So, he shoots him in the head. It's obvious that West's gun doesn't fire, and the soldier just pretends like he's been shot.
Revealing mistake: During the big final fight at Midian, at one point, one of the men with a flamethrower accidentally stumbles backwards into a patch of burning shrubs/plants, which sets him on fire and eventually causes his flamethrower tank to explode. As he accidentally stumbles back into the shrubs, watch closely- one of the plants starts to fall over, and is attached to a small square board hidden in the dirt below that becomes visible, revealing that the plants were pieces of set-dressing.