Continuity mistake: When Sarah is injured and riding up in the dumbwaiter with Charlie's dead body the position of Charlie's head changes significantly between shots. (00:59:57)
Continuity mistake: Just before Michael stabs Laurie you can already see the injury make-up applied to her arm as she swings the axe at him. (01:10:39)
Continuity mistake: When Sarah is in the dumb waiter with Charlie, she looks down at the cut on her leg, then at him, and he's looking straight forward and up. In the next shot, as the dumb waiter reaches the top, his head is turned, looking at her, and remains that way until she exits and the dumb waiter drops when Michael cuts the rope.
Chosen answer: There is no answer to that. Its simply bad work by the writers. They never tried to explain that. You could perhaps assume shock had prevented the paramedic from immediately taking the mask off. Unlikely though,.