Revealing mistake: When Charlie turns and sees Michael after futzing with the garbage disposal, it's obvious that Michael's mask is a (rather poor) CGI creation. It looks very cartoonish and unlike the way it looks in the rest of the film. The scene was shot early in production with an original mask that eventually was tossed out for the final design when producers thought it didn't quite look right. Because Michael was in a medium-closeup shot and they didn't have time to reshoot the scene, they opted to (somewhat obviously) digitally replace the mask.
Continuity mistake: When Will and Laurie are in the hall, it is Michael who comes up the stairs, as you can see the mask and hair. It's not until they cut away to Will and Laurie, then back again, that "Michael" becomes LL, and gets shot.
Continuity mistake: In the yearbook photo of Laurie it says class of 78. She just started her senior year a few months earlier, so she would be class of 79.
Character mistake: In the beginning where they are showing all the newspaper articles, the article uses Michael "Meyers" twice in one article and then back to "Myers."
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)
Visible crew/equipment: When the school buses depart from the school for Yosemite you can see the reflection of the crew's various tents and facilities in the buses side windows as they drive past the camera. (00:42:43)
Visible crew/equipment: After discovering John and Charlie in town Laurie drives them back to the school. The front gates are closed so Laurie honks her horn for the security guard's attention. John then get's out the car and as he does so you can see some equipment reflected in his car door just off-screen. There's some kind of large blue object and orange traffic cones definitely visible. None of it is there in wide shots. (00:33:46)
Visible crew/equipment: The first time we are introduced to Charlie and Sarah they are discussing the trip to Yosemite with John. As they walk through the school courtyard pay attention to the glass windows behind them (right side of the screen) and you can see a reflection of the camera and crew following in front of them. (00:17:59)
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,.