Revealing mistake: When Carl is being dragged away by the demons, you can see the cable line that he's attached to. May be fixed on DVD.
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 "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: 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: In the scene where Kim comes home early from the camping trip, Edward is startled and begins stabbing the water bed, thus causing it to spring many leaks. If you actually look at the bed, it isn't jiggling like a water bed should and the water is just spraying out even in places he isn't stabbing.
Revealing mistake: When Larry is camping in the tree outside Ellen's house, if you look at his face before he falls it is a stuntman falling, not James Belushi. (01:03:35)
Revealing mistake: Right after Xayide dies we see a shot of two monsters. One of the monsters explodes and the particles from the explosion hit the other monster making it break in half before it explodes itself. (01:15:25)
Revealing mistake: When the car crashes into the tall display of tennis balls in the store, there's a dummy on top of the car dressed up in the same clothing as Ben.
Revealing mistake: The bottom of Tony's legs disappear when Elliot emerges from the phone and grabs hold of him.
Revealing mistake: When Webby and the others are standing outside of the window watching Scrooge pace back and forth in anger, she casts no shadow on the patio. This is revealed when she moves left at one point. (00:28:50)
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.