Other mistake: When Doc uses a computer to watch/simulate dog cells being assimilated by a "thing" cell, we can see a single cell fusing with multiple dog cells to imitate them. This process would lead to the dog being digested until it remains only one cell, and not to the replacement of all of its cells by the imitators. (00:40:30 - 00:41:25)
Suggested correction: The computer simulation isn't showing just one cell taking over an entire dog, but showing how the creature can get the genetic makeup of whatever it touches and replicate it perfectly.
I think it's fair to consider this a goof. John Carpenter states on the director's commentary his goal through this sequence was to demonstrate the life cycle of the Thing, and acknowledges that the visual isn't accurate for that purpose.
Pretty much the entire rest of the movie unfolds as though the simulation showcased the Thing spreading / multiplying: it's followed by text saying the entire human population could become "infected" after a certain amount of time; it's not until after this scene that anyone besides Blair is worried that one or more of them has been taken over. It's a valid movie mistake because the movie itself seems to assume the audience saw something different than what was actually shown.
