(at around 1h 23 mins) After Palmer's blood leaps from the dish after being scorched by Mac's flamethrower, we see Palmer (David Clennon) morphing into a Thing-monster. Several men tied to a couch are now screaming in terror for their lives. We hear Nauls's (T.K. Carter) screaming loudly but when the camera catches some shots of Nauls, he's just sitting their placidly when the soundtrack simultaneously reveals him to be screaming.
(at around 1h 6 mins) When Mac yells to Norris in the rec room, "Any of them move you fry 'em," a shadow cast by the crew's boom mic can be seen on the wall above Clark, Copper, and Garry on the couch.
When Macready and the doctor go off to check the Norwegian base, it is said they should only take about an hour to get there, and it is light when they leave. However, there is a night-time scene set at the American base in-between the time it takes for Mac to arrive at the Norwegian base, even though it is daylight when he arrives there.
All exterior shots of the tool shed where Blair is imprisoned show the window next to the door has been boarded up with the boards horizontal. However, all interior shots of the shed show that same window with the boards attached vertically.
(at around 1h 15 mins) When the Doctor goes to hit Norris with the defibrillator the second time, his hands change position as the camera cuts from one angle to another. As he starts the move the paddles are coming down side-by-side on the man's chest, just like the previous time. The camera cuts to a close up, an effects shot where the chest cavity opens up as a sort of mouth and swallows Dr. Copper's arms. This time the paddles are coming down laterally aligned with Norris' sternum, rather than side by side, as they started.
(at around 18 mins) At the Norwegian base, when Mac and the doctor enter the room with the ax in the door, you see the door closing in the background, but in a later camera shot, you see the door still wide open.
(at around 1h 5 mins) When MacReady visits Fuchs in his room, and Fuchs mentions preparing their own food, et cetera, there is a beaker of dark brown liquid on his desk. When the lamp flickers we get a close-up of the beaker and it now contains clear liquid.
(at around 1h 13 mins) As Mac comes out of the supply room with the flare and dynamite, the TNT sticks he is holding are almost falling apart and he is holding them by one stick. In the following shots of Mac the TNT sticks are suddenly wrapped tightly with black tape again and he holds all the sticks.
(at around 8 mins) When the helicopter explodes there are almost no parts near it. Later while it is getting extinguished, there are many small parts burned near it. Also the whole main rotor-blade is missing. It was on the helicopter just after the explosion.
(at around 12 mins) When the men are gathered around the dead Norwegian, Fuchs is seen standing among the group and Palmer is also there offering Copper a ride. In the very next scene, Copper is walking towards the helicopter wreck telling Mac to get his things on and Fuchs and Palmer are already standing by the wreck dressed in their snow parkas.
When MacReady is administering the blood test with the hot wire, we see the characters cut their thumbs and let some drops of blood into the dishes. However, when he glances down at them a lot more blood has appeared in some of the dishes.
We see McCready drinking at the beginning of the film. He is shown in a camera shot from below and we see what appears to be a long, messy, shaggy beard. A few minutes later when he gets into the helicopter his beard is neatly trimmed and combed.
(at around 55 mins) When Mac is in the tool shed talking with Blair, the bottle of vodka from which he drinks mysteriously spins 180 degrees. He sets it down with the label facing the camera but in the following shots the label now suddenly faces away.
Between the time Fuchs talks to Mac about Blair's notes, calling the snowcat 'the Thiokol' (another brand of snowcat) the logo and model changes from a Skidozer 301, with its model name between the headlights, to a 302, with the Bombardier lettering between the headlights and the model name lower down to the left side (screen right). When they take Blair out to be locked up, the snowcat is again a 301 (you can see the blue model-plate between the headlights, not the black-on-yellow of the 302's Bombardier lettering).
(at around 5 mins) When MacReady plays Chess Wizard and spills his drink into the circuit board, we see one big ice cube. The problem is he had taken three small ice cubes for his drink, which had also almost fully thawed right before he spilled it.
(at around 1h 17 mins) When the "Head Bug" is shown trying escape the facial features still retained by creature are facing away from the group as it crawls toward the door. However in the very next shot just before Mac torches it the creature has suddenly stopped running and facing into the room.
(at around 9 mins) Macready's bottle of whisky is completely covered in snow when he leaps into the snowdrift as the chopper explodes, but when he runs to help Bennings in the very next shot, it is totally free of snow.
(at around 52 mins) When MacReady goes to check the helicopter after he sees Blair, he is wearing a protective hood. After he hears the gun shot and runs out of the helicopter, his hood has disappeared.
When Norris and Mac climb down into the crater and are walking towards the open hatch in the center, look on the far right hand side of the screen and you will see the head and arm of somebody come into shot and then disappear again.
(at around 15 mins) Right after Bennings yells on the intercom to turn the music down and we see his request ignored by Nauls in the kitchen, the following scene that slowly dollies from right to left in the empty medical room, the moving wheels of the dolly and a man in a light blue shirt pushing the dolly are clearly seen in the reflection of bottom and top pane of glass of the stainless steel and glass medical cabinet. Especially visible on the Blu-ray version.
(at around 1h 30 mins) As the men come down the stairs into the generator room, Nauls and Garry head down a second flight of stairs as Mac holds up a flare. The shadow of a crew member is clearly visible on the floor holding a boom mic. The shadow even moves as the men walk away from the camera.
During the many interior scenes where the flamethrowers was used, resulting in large fires being started and then extinguished by the characters, the rooms the fires were in would be left with thick choking & blinding smoke, yet you see the rooms are always smoke free after the fires are put out.
(at around 5 mins) The chess game McReady plays vs what appears to be a dedicated chess-playing computer (ie a computer built specifically to play chess, period) is totally bogus. There is no "check" much less checkmate possible in the position we see immediately before McReady is "checkmated" there is no "Rook to Knight 6" with check possible and even that's inaccurate - correct is "Rook to King or Queen) Knight Six" although even using English Notation was dubious - chess programs use Algebraic Notation.
(at around 44 mins) Macready says "It's forty below outside." That would be a freakishly warm winter day in inland Antarctica, where winter temperatures are closer to minus 80 Fahrenheit (minus 62 Celsius).
The overwhelming majority of people from Norway are taught English from a young age, and as a result, most Norwegians can speak fluent English or at least know very basic English vocabulary. In reality, it's practically impossible that there would be a language barrier between the Norwegians and the Americans like shown at the beginning of the film.
Flamethrowers would be useless in Antarctica, especially outside. Gasoline (even in the form of napalm) has too high a freezing temperature to forced out of a tank without solidifying. While there may be heating torches used, these would probably be LNG (liquified natural gas) and not napalm. They also wouldn't shoot flames out as far as a flamethrower.
The alien Norwegian dog is caged with the American dogs and starts mutating while attacking the other dogs. When the crew check out the ruckus, first seeing the mutated dog, they start their counterattack and the alien jumps up to the ceiling of the cage. While this is happening, in the audio of barking dogs and people shouting, you can hear the sound of a rope or chain pulley in operation to raise the creature up.
When Mac, Garry and Nauls are underground, they split up and go alone to separate areas to plant the explosives. They do this instead of staying together despite knowing full well "The Thing" usually attacks a human while they're alone.
While burning the creatures might destroy the outside cells of what they were imitating, it would do nothing to destroy the cells on the interiors of the body. In fact, the body would act as an "insulator" protecting the inner cells from the excessive heat. Doc would have realized that and made mention of it.
(at around 1h 23 mins) When one of the characters is slammed into the ceiling, a piece of the ceiling breaks off, but instead of falling to the ground, it "falls" upwards into the ceiling, revealing the scene was shot upside-down.
(at around 8 mins) When the Norwegian is firing on the American scientists at the beginning of the movie, we clearly see that Childs has already jumped into the snow in previous takes of the scene. We can see a shape on the snow.
(at around 39 mins) When MacReady and Norris are approaching the hole in the ice next to the spaceship, Norris's head is partially obstructed by an obvious matte-line during the dissolve-in part of the establishing shot.
(at around 53 mins) When Blair is destroying the radio room he drops the axe to the floor and fires his gun. When they rush him with the table he suddenly has the axe back in his hands and hits the table with it.
The alien dog apparently runs from the Norwegian science station to the American station. Considering that helicopters, and not snow cats, are used to travel the distance between the two, it's likely that they were at LEAST 75-100 miles apart. How did the dog make it that far (or farther) simply being a "dog" and why wasn't the much faster helicopter able to land ahead of the dog and wait for it?