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.
Visible crew/equipment: When the "Palmer Thing" attacks Windows it lifts him up in the air. As it does this you can easily tell the monster is played by a member of the stunt team by the dark green T-shirt it's wearing. Palmer's clothing is different before and after this shot. (01:23:46)
Factual error: If you are at or near a pole, aka Antarctica, you either have 24-hour daylight, 24-hour night, or a kind of continual dusk-like condition. You don't have bright day and dark night at the same time.
Character mistake: The spaceship is much deeper in the ice than frozen thing, indicating it was buried a hundred thousand years after the spaceship landed. Norris' calculation of the age of the spaceship wouldn't apply to the thing, which was hardly buried at all, by comparison.
Other mistake: Blair somehow manages to fire seven shots with his six-shot pistol.
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)
Visible crew/equipment: When the "Thing" monster bursts out of Norris' chest it rises up to the ceiling and grabs onto an oblong-shaped ventilation shaft. MacCready steps forward with his back to the camera (getting ready to burn it) and in this shot if you look at the adjacent pipe on the ceiling you can see the hand of a crew member. It looks like the crew member is there to steady or grab the monster puppet if it falls. (01:15:45)
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