Factual error: When the infected Palmer starts his transformation into the Thing, he has a series of fits and the creature 'falls', or leaps upward onto the ceiling. Watch closely...you can see a piece of paneling also 'fall' upward, when it should have fallen down. (01:23:20)
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.
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