Revealing mistake: When the chair moves across the kitchen floor, Craig T. Nelson goes to investigate it (after it has stopped moving). He goes up to the chair and flips it over. If you look closely, Craig almost hits the camera with the chair's legs. You can tell this has happened because the camera moves up very quickly and Craig looks straight at the camera crew with very big guilty eyes. This is more easily visible on the widescreen version of the film. (00:33:30)
Revealing mistake: When the Enterprise is leaving space dock, look at the right (starboard) side of the ship. You can see the shape of the mounting arm the model was mounted on, even though it was blacked out to look like space, and it also blocked out the bottom of the dock. The footage was reused from Star Trek: The Motion Picture.
Revealing mistake: When the oldest son is being pursued by the unseen entity whilst the family is seeing the priest. You can see a crew member in green pants and brown shoes to the left as the camera perspective turns upside down.
Revealing mistake: After the snake lady is dead, eyes open, Deckard rolls her head from camera right to camera left. Her eyes move. (00:59:15)
Revealing mistake: When Rambo throws a rock at the police helicopter and knocks Galt out, you can clearly tell it's a stuntman. Galt is in his 40s and overweight. The guy that was falling looked like he was 25 and was noticeably thinner.
Revealing mistake: When MacReady burns the one "Thing" with the split head, in one shot it walks by MacReady in the foreground, and you can tell in that shot that it is just a still dummy burning, as it isn't moving at all. It just sort of floats past with its arms at its sides, while in the previous and next shots it is wildly moving around, arms flailing.
Revealing mistake: When the bugs crawl out of E.G. Marshall's mouth, it is obviously a dummy.
Revealing mistake: The dead woman who is found at the beginning of the movie takes a rather deep breath at the end of the shot. (00:02:40)
Revealing mistake: During the Eddie and Nick fight scene, you can clearly see the stuntman for Eddie. He looks nothing like him.
Revealing mistake: After Flynn, Tron and Ram take their drink, they resume their journey in their stolen Light Cycles. There is a shot of them coming around a corner and riding along a cliff side. If you look carefully (this is painfully obvious if you go frame by frame) they don't actually come around the corner. Their Light Cycles simply appear one by one and they make their turn. (00:50:40)
Revealing mistake: At the end when Mr Norris starts to run through the hallways looking for the punks, he is ambushed and beaten up. If you look closely at his shirt, you can see a small pad underneath on his left, the exact spot where they keep punching him.
Revealing mistake: During the autopsy scene, it's obvious that the Panther is still breathing when it's supposed to be dead.
Revealing mistake: After the real Leon Sprague is clubbed to death, Pavel Upenskoy and his accomplice toss him into the nearby river. Just before he is dumped, the corpses left hand flexes open and bends upward towards Upenskoy's head.