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: 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)
Continuity mistake: When MacReady and the two other researches first discover the the spacecraft, there is a shot where the camera is in front of the three looking downwards into the hole. All three of them have their hands by their sides. In the next shot, the researcher on the left suddenly has his arms raised with binoculars suddenly in his hands. (00:38:20)
Continuity mistake: After the Norwiegans' helicopter has chased the dog-Thing to the American research station, the chopper flies around the station before landing on the ground. Before landing, MacReady is shown exiting the watch tower with a bottle of alcohol in hand. When he exits, the bottle is around his hip area. Then from an angle of MacReady low down, the bottle is suddenly raised slightly up to his chest area. (00:06:35)
Continuity mistake: Just after MacReady leaves Fuchs to do his work, there are several items on his desk, such as a lamp, conical flask and books. In the first shot shown after MacReady leaves, the lamp is about a foot away from the conical flask and the conical flask is a foot away from the books. Next shot, the lamp, conical flask and books are more closer to each other. (01:05:10)
Continuity mistake: In the opening scene, a Norwegian helicopter is flying across the Arctic. The co-pilot is half outside it. From the exterior angle, the co-pilot has a pair of binoculars and is looking directly downwards with the binoculars also positioned directly downwards. In the next shot, from a side angle, the binoculars are suddenly elevated and he is looking directly forward. (00:02:55)
Continuity mistake: When MacReady is recording the stuff he knows about the thing, a glass of alcohol is on top of the tape recorder, located near the front of it. The camera then changes to a close-up of the recorder and the glass is now near the back of it. (01:04:00)
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: When MacReady and Doc are examining the abandoned shack, MacReady comes across a portable video unit and MacReady then wipes off the snow and dust off the unit. From the first shot of the unit, we can see part of the unit is wiped so far, we are able to notice the unit itself. The visible part of the unit is located in the middle of the unit at first, but after Doc states the papers are in Norwegian and the shot cuts back to MacReady, it is now the edge of the unit that has the top of the unit visible. (00:20:00)
Continuity mistake: MacReady leaves the room so Fuchs can continue to do his work. The shot changes to an angle from behind Fuchs where he is shown writing on his paper. There is a conical flask visible at the back of the desk with some blue liquid in it. In the next shot, the liquid in the conical flask is suddenly just clear liquid. (01:05:10)
Continuity mistake: When MacReady tests Naul's blood sample, in the close-up shot, the items on the cardboard box change by comparison to the wider shot before. The revolver moves and the bloody scalpel appears from nowhere. (01:25:21)
Continuity mistake: When in the helicopter, the Norwegian men are both wearing different types of goggles, but when they're out of the helicopter, they both have the same type of goggles.
Continuity mistake: When Palmer is revealed to be a thing, he shoots up to the ceiling. Windows is ready to use the flamethrower on him. He has the flamethrower pointed up to the ceiling, yet when Palmer falls to the floor, in that split second, Windows changes into a completely different position with the flamethrower. (01:23:35)
Continuity mistake: When Childs takes an ax to the supply room door where MacReady has broken into, there is a shot of him hitting the door high up. Then there is the shot of MacReady almost frozen. When the shot cuts back to Childs, the ax marks in the door are not consistent with the shot from a few seconds before. (01:12:50)
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