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.
Continuity mistake: When the stray dog is placed in the cage with the other dogs and starts to mutate into The Thing, one of the other dogs start biting a hole in the fence. We get a clear view of the hole in one close-up, and in the next shot as The Thing sprays liquid on the dog, and the hole in the fence is much bigger. (00:27:45)
Continuity mistake: When the remaining men find Macready holding a flare to a bundle of sticks of dynamite, the sticks go from being loosely and haphazardly bundled together to tightly tied together and then back again in a matter of seconds. (01:12:45)
Other mistake: When Blair goes crazy with an axe, they put him in the tool shed and Doc injects him with something to calm him down. The problem is: Doc injects him in the inner elbow, but wipes the upper arm as if that's where the injection went. Plus you only inject in the inner elbow when you want something IV, but there was no tourniquet, no searching for a vein. It should have been given IM, into a muscle, like the upper arm.
Other mistake: When Doc's arms are being ripped off by Norris' chest, his left arm is cut by the teeth but his right arm is pulled off above the wound like rubber. (01:13:00)
Continuity mistake: After the Norwegians exit the helicopter, one of them goes to throw a grenade but accidentally lets go of it, and it explodes. In the shot where the man takes cover during the explosion, the fire clears out and there is soot around the helicopter on the snow, about six feet. A couple of shots later, as the Norwegian stands up, the amount of soot around on the snow has notably increased. (00:08:20)
Visible crew/equipment: When the "Palmer Thing" attacks Windows it lifts him up in the air. As it does this you can easily tell the monster is played by a member of the stunt team by the dark green T-shirt it's wearing. Palmer's clothing is different before and after this shot. (01:23:46)
Continuity mistake: MacReady and Doc return to the research station from the abandoned shack in the helicopter. The scene then cuts to the dog watching the two exit the helicopter in the window. In the shot where the helicopter is visible in the window, the dog is looking out of the window from the right hand side. In the next shot, it is suddenly looking out of the left hand side. This mistake has nothing to do with the angle of the camera. (00:22:35)
Continuity mistake: After MacReady and Doc have discovered the spacecraft, the scene cuts to a wide angle of them approaching one of the researchers looking into a dug up hole. Just as the wide shot ends, MacReady and Doc are standing right next to the man. The shot then cuts to a closer angle of the three and MacReady and Doc are now several feet away from him. (00:39:50)
Continuity mistake: After the scene where the Norwegian helicopter chasing the dog through the snow ends, the scene cuts to the research station. The first shot from the research station shows a sign: UNITED STATES NATIONAL SCIENCE INSTITUTE STATION 4. This close-up shows scratching and rotting all over. A few minutes later, as the helicopter arrives at the station, the scratching and rotting over the "4" is no longer there. (00:04:30 - 00:06:15)
Continuity mistake: When the Norwegian helicopter finds the dog running in the snow, the dog turns around to notice the helicopter, causing some of the snow around him to push upwards a few inches. The shot cuts to the helicopter, but when the shot cuts back, the snow is suddenly almost level with the rest. (00:03:35)
Continuity mistake: When MacReady says they are going to find out who's who, he tells Doc, Garry and Clark to move away from the others and one of them steps forward in MacReady's direction. MacReady then raises his flamethrower around the top half of his abdominal region. The shot then changes where the camera is in front of MacReady and the top half of his abdominal region is shown, but the flamethrower is nowhere in sight. The camera cuts to an angle behind MacReady and the flamethrower is back. (01:02:10)
Continuity mistake: When Windows breaks the glass to the gun cabinet, he takes out one of the shotguns and picks up some ammo. At the left hand side of the cabinet, there is some remaining glass in the cabinet along the side. The glass along the left side vanishes when Windows places the shotgun on the floor. (00:59:15)
Continuity mistake: Back at the research station, the researchers are looking at some debris from the spacecraft that is placed on a cloth on a snooker table. There is a guy with a grey sweater with his hands in the debris and a guy in a brown jumper also holding the debris. In the next shot, neither of their hands are located on the debris or cloth. (00:40:50)
Continuity mistake: When Doc is defibrillating Norris, in the shot before he defibrillates him and we see it is actually the thing, Norris' eyes are definitely closed. In the next shot, just as Doc bursts through Norris and the Thing's intestines are revealed, Norris' eyes are suddenly open. (01:15:20)
Continuity mistake: Just after MacReady overhears the dogs barking, he runs over to the fire alarm, breaks the glass with his beer can and pulls the fire alarm. After he breaks the glass, there is a piece of broken glass located next to the alarm. But in the next shot, as he removes his hand, the piece of glass has vanished. (00:30:10)