Plot hole: It's never explicitly stated or shown that the Thing reproduces with each victim until the movie is nearly over (when Palmer infects Windows). Most viewers figure it out from the context, but it's unclear just when and how the characters themselves have come to this conclusion. This was an inadvertent result of an editing decision and a visual goof: there is a deleted scene in which Blair explains much more directly that the Thing multiplies according to how many victims it takes, and in its place in the final film is a scene containing a computer simulation that director John Carpenter acknowledges was a failed attempt at explaining the organism's life cycle.
Plot hole: As the gun-toting Norseman approaches the buildings, Garry smashes the single-pane window with his handgun. It is inconceivable that the glazing in a structure near the South Pole would be single-pane glass, that could be broken so easily.
Other mistake: The brief chess game (four moves) shown in the beginning of the movie makes absolutely no sense. For instance: There are no white bishops on the board, but after the human player taps a few buttons, the computer says that he just moved one. Within the two moves described first, the positions of the pieces change completely. In addition, some pieces are now gone and there are new pieces on the board. It's like an entirely different game. The final move is said to have been made with a black rook, which is impossible. In the shot shown before this move, there is not a single position available to that chess piece that would result in an instant checkmate. Nothing in the editing indicates that these few short shots are supposed to be viewed as separate games. (00:04:50)
Character mistake: When MacReady is giving the blood test, the scalpel they are using is not properly sterilized after each person cuts themselves (shown when Nauls cuts himself, then when Windows goes next: all he does is wipe the scalpel on his pants). Using the same scalpel seems like an extremely stupid thing to do as Fuchs told MacReady that even a small particle of the Thing could take over a person. (01:19:55)
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.
Visible crew/equipment: After MacReady says, "The generator room," toward the end of the film, he, Garry and Nauls walk down a flight of stairs. Then Garry and Nauls walk down a set of three steps whilst Mac is holding a flare. As they walk the shadow of a guy holding a boom mic is clearly visible on the floor. (01:30:05)
Visible crew/equipment: At the end of the film, where MacReady is about to fight 'the thing' you can see the shadow of a guy holding a boom mic, on the ground just behind him.
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: When the dog, aka the The Thing, is led into the dog pen for the first time we see the floor is covered in a thick layer of straw for the dogs to sleep on. This is seen every time except when Childs uses the flame thrower. We get a wide shot of the pen as it's lit up in flame and the floor is now completely bare of any straw. There are also none of the recently deceased dogs that were lining the floor, being converted into Things. (00:31:05)
Continuity mistake: Just a couple of minutes into the movie, Gary shoots dead the crazed Norwegian and the camera returns to MacReady. Now watch closely: When he gets up to help injured Bennings, he and his Jim Beam bottle are covered up to the hilt with loads of snow. But on the next shot, a split second later, his clothes are miraculously clean and the whiskey bottle as dry as you can possibly get it. (00:09:00)
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 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)