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: 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 the little girl runs towards her grandmother and knocks another old woman out of the way, the other woman crashes into the wall of the terminal and it wobbles, showing it's a thin set wall. (00:14:35)
Revealing mistake: When the boys are racing away on their bicycles from the Feds, the stunt doubles for Elliott and his friends are obviously way too tall to be children.
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: During the part when the day reaches October 31st, various scenes show children and happenings with their Halloween masks throughout various portions of the country getting ready for the big give-away. Included with this is the scene showing Dr. Challis' two kids again sitting in front of the television with their masks on, but they are wearing the same exact clothes they had on when he had stopped over at their house 8 days earlier. This scene was obviously cutting-room floor from that previous scene.
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: When the Swamp Thing walks past a bush, it's camouflage netting.
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.