Towards the end of movie the Thing breaks down door and enters barracks and is doused with kerosene and lit on fire. As soon as fire is lit, it is obvious the stunt man is not only shorter, but he is wearing a bulky asbestos suit, not the tight one James Arness is wearing as the Thing.
At the conclusion of the scene where Cap. Hendry first meets Dr. Carrington, as they are leaving the lab, Carrington refers to Nikki as "Miss Litton" rather than "Miss Nicholson." Margaret Sheridan's face registers her awareness of the actor's mistake, but she "goes along with it" and the scene was printed "as is" rather than give a retake.
Carrington describes the "Pliocene" human-ancestors as being primeval worm-like creatures. This description would be better suited to the Cambrian or Paleozoic times. In the time of Pliocene (Greek for "beginning of the recent"), recognizable man-like creatures were already showing up. The australopithecines came around during the later part of this time.
When the men arrive at the crash site and are looking it over, their shadows are always pointing away from us no matter whether we are looking at their faces or looking over their shoulders at the site.
Near the end of the film, when the Thing breaks through the door to confront the airmen for the last time, the wood planks that had been used to reinforce the door are broken off and dispersed. Then the camera quickly cuts to the airmen and then back to the Thing. Only now there is a new large piece of wood that appears next to the Thing, which he proceeds to pick up. This piece of wood was not there when he initially broke through the door.
In the credits there is a listing for "Lt. Ken Erikson," but he's introduced to the reporter as "Lieutenant MacPhearson, " and is continually referred to throughout the film as either "Mac" or MacPhearson.
When the C-47 transport first lands at the base, the plane comes to a halt, the propellers stop, the door opens, and the crew and passengers climb out. Not only is one of the propellers suddenly in a new position, but there is also a buildup of snow on the wing that was not there a moment before.
When Bob the crew chief fires his gun at the greenhouse door. Six bullet holes appear in the door. But there were only three shots fired - 3 muzzle flashes and 3 ejected cartridges can be seen coming from the gun.
At the end, when the Thing is walking on the platform, there are tall, thin boxes along the right wall. When the 'switch' is thrown, they are replaced by 2 large square crates. Also, when he drops the 4x4 off the platform, it reappears back on it, gets nudged slightly, then is back off it at the end.
When the airmen douse the Thing with kerosene, two buckets hit him while he is standing clear of others. The Thing then moves toward the door where Nikki is manning the light switch and a third bucket is thrown at him. The kerosene that doesn't hit the Thing splashes, igniting the wall, floor and desk, all around where Nikki is supposed to be standing. After the Thing leaps through the window, only the mattress Nikki used as a shield is burning, the room behind her shows no sign of fire damage.
As they fly over the crash site, you can see a small black squarish-object at the far right of the screen on the ice. This could be some piece of studio equipment, and it certainly doesn't belong there.
The captain states that he is navigating to the crash site near the north pole by steering toward "that peak ahead", and black mountains can be seen on the horizon in the scene of the crash site. There is no land closer to the north pole than Kaffeklubben island, 707 km (440 miles) from the pole.
The location being the North Pole, there would be no day/night cycles. Further, given the date of November 2 (from Dr. Carrington's log entry), the North Pole would be over a month and a half into a 6-month period of darkness.
At the beginning of the picture the base is identified as being 2,000 miles away. The aircraft is a C47 (2-engine Dakota) with an absolute maximum one-way best circumstances range of 1,600 miles. The 400-mile shortfall is roughly the air distance between Philadelphia and Montreal.
The block of ice that contains the Thing they chop out would have to weigh close to a ton, and therefore near-impossible to lift out of the ice, or have the dog sled move it easily, or lift it into the plane.
In the scene during which a soldier mistakes the 'radioactive isotopes' in a storage room for signals being emitted from the Thing, an electrical wire providing electricity to the Geiger counter can clearly be seen coming out of his right pant leg.
The first close view of the creature behind the door of the greenhouse is exiting and powerful, with wood chips flying everywhere when he takes a swing at the cast. But when viewed in slow motion it's obvious that his hand comes nowhere near the door and an explosive charge is used to blow out the wood towards the camera. Similarly when he draws his trapped hand back inside, chips blow outward and not inward as they would naturally.
The goof item below may give away important plot points.
Towards the end of the movie, the team barricades The Thing in the mess room with wooden boards. When the creature finally emerges from the mess room for the final confrontation, the door swings inwards, towards the creature, obviously opened by James Arness. Therefore, the barricades, which would have prevented the door from being opened outwards, were useless.