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)
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)
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 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 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 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 Palmer, as the thing, picks up Windows by the head his wardrobe is different as soon as he picks him up. The sleeveless motorcycle jacket he is wearing is missing and he is just wearing the green short sleeve shirt and the white sweatshirt. Then in the next shot he has the sleeveless jacket on again. (01:23:40)
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 they are all outside and MacReady has separated the three most likely suspects and orders that they be tied down and tested, he comments at that time that they are to be hit by a storm in six hours. Cut to the scene where Mac is dictating into his recorder and he comments that the storm has been hitting them hard for 48 hours. However it is the same night and at best, maybe only an hour has passed.
Continuity mistake: When Mac is locked out of the complex by the rest of the team, who think he may be a 'Thing', Childs uses an axe to try and chop open one of the doors, leaving a HUGE hole in the middle of it. Later Childs returns to that same door and the hole is significantly smaller and at a completely different location on the door.
Continuity mistake: When the crew is outside and they're going to lock Blair in the tool shed, you can see someone nailing wood across the window on the front of the shed. The pieces of wood are horizontal. In the next shot (inside the shed), if you look at the window in the background, the strips of wood are vertical. When Mac goes back outside, the wood is horizontal again.
Continuity mistake: Each man only dribbles a few drops of blood into the petri dishes as their blood samples. But when the test is administered, there is a lot more blood in each dish, enough to cover the entire bottom.
Continuity mistake: When the Doc and Kurt Russel enter the burnt Norweigan building the door closes itself behind the Doc. It closes and opens several times over the course of the scene.
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 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.
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.
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