Revealing mistake: When Luke is being fed to the Rancor, in Jabba the Hutt's dungeon, there are black outlines around the beast's legs, from the composite's blue screen special effect. This was edited out in the special edition rerelease.
Revealing mistake: In the scene when Christine smashed repeatedly into the silver Camaro at the gas station, there is no motor in the Camaro. The hood of the Camaro gets knocked off the hinges and is twisted sideways on the car. As Christine is in reverse and dragging the Camaro along you can see down through the hood that the engine compartment is empty. Minus a transmission also, I believe. (01:15:30)
Revealing mistake: Just after Mr. Creosote explodes and all of the guests are vomiting, there is one male guest third from the front that is puking. Though his hand is up near his mouth, it is clear that the puke actually comes from his hand rather than his mouth, revealing that he is holding something that produces the fake vomit.
Revealing mistake: The outline of the blood pack is visible under John's shirt when Stillson's body guard shoots him. (01:37:35)
Revealing mistake: When Superman gets blown away by the bombs, he is being smashed onto the rocks. You can easily see that the rocks aren't real, but also that when he rises up again, you can see the cardboard rocks dent.
Revealing mistake: When Passenger asks Driver to pull off the side of the road, so that he can "show him something really scary", Driver pulls way off to the left shoulder, instead of just pulling onto the right shoulder.