Continuity mistake: When Doc is trying to revive Norris and his arms fall into Norris' chest, he is wearing long sleeves down to the wrist. In the closeup of Doc's arms being ripped off, the sleeves are pushed up to the elbows and the forearms are bare. (01:15:15)
Continuity mistake: When the three men are shown finding the UFO from the front, their hoods are down. In the next scene, from behind, their hoods are over their heads.
Continuity mistake: When Kurt Russell first looks at the chess game, amount of ice in his glass changes between shots.
Continuity mistake: When the thing is on the lab table, and the severed head starts walking away, Mack torches it with a flamethrower. At one angle, you can clearly see that fire is not coming out of the hose. It is just a fire going off in the doorway. (01:17:20)
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)
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)
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.
Factual error: At the beginning, MacReady dumps a tumbler of ice and scotch into his computer, which spews a shower of sparks as it shorts out. No electronic device, not even an old 1980s personal computer, sparks like that when shorted out. It simply goes dead.
Visible crew/equipment: When we see the huge dog alien, right before Childs burns it, a crewemember controlling the puppet is seen on the center of the monster's head.
Continuity mistake: When Kurt Russsell first goes to the crazy guys' shack, he puts a bottle down with the label facing forward. When he leaves it is facing the other way.
Deliberate mistake: When the Norwegian starts shooting at the dog, Childs jumps into the snow and you can see the shape before he jumps in where he's had to do this before on different takes. (00:08:40)
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)
Continuity mistake: When the helicopter is circling the research station, the co-pilot picks up a hand grenade to throw on the ground. He picks up the grenade on the bottom row in the middle column. There is no grenade in the middle column from the second row of grenades and no grenade in the left hand column of the top row of grenades. Around 45 seconds later, in the shot after the dog leaps onto one of the researchers, the exact same shot of the man taking the grenade out of the box has been repeated a second time. He takes out the exact same grenade from the exact same row. (00:07:15 - 00:08:05)
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)
Answer: The death of Fuchs is probably the best answer to this question, as it appeared that Fuchs burned himself alive before the Thing could assimilate him. It's also possible that Norris suspected he was infected before he transformed. There is a scene in which we see Norris, who is alone, suddenly wince in pain, surprised, and grab at his chest, but he continues functioning normally thereafter. Following the altercation with Mac, Norris collapses and becomes unresponsive, until his chest cracks wide open and bites off Copper's arms. Also, in the blood test scene, Palmer's facial expressions appear to betray his secret, but he was already fully transformed at that point.
Charles Austin Miller