Continuity mistake: At the start when Jimmy opens the fridge door there are quite a lot of items in there. If you look to the right of the beer bottles on the lower shelf there is nothing but a purple box. In the next shot when he starts to take the beer out of the fridge you see that on the lower shelf beside the beer bottles there is a red object. (00:04:00)
Continuity mistake: Jimmy opens the fridge. On the second shelf there are no packages of lunch meat. A few seconds later a pack appears, and shortly after that 4 more appear out of nowhere. (00:04:10)
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)
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: 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)
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: Near the end of the film Keri/Laurie stabs Michael with a flag post through his chest and when he falls over his mask is different and his knife is much bigger. (01:09:15)
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 Jamie Lee Curtis is teaching a class, just prior to the class trip, a shot over her shoulder shows on the blackboard 'W.B. Yates'. It should be spelled 'Yeats'. In a shot a few seconds later the spelling has been corrected.
Continuity mistake: When John dives out of the window after first seeing Michael, there is no way he can land on his feet. He dove out head first.
Continuity mistake: More than one mask was used in the film. This is most noticeable as the look of the mask changes from time to time.
Continuity mistake: In one scene, Jamie Lee Curtis takes a bottle of something out of the fridge, removes the top of the bottle and takes a drink, then hands the bottle over to her new boyfriend, he also takes the top off the bottle, even though it was never replaced - Jamie Lee Curtis can be seen playing with the top in her hands.
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 Jamie Lee Curtis goes back into the school with the axe and Michael Myers is hanging on to the pipe you can see she has a cut on her arm. When Michael comes down he stabs her in the spot where the cut was - you can clearly see it has disappeared.
Continuity mistake: When the mom and the little girl go into the men's room, the mom blocks the door with a rock. If you look closely the rock changes angles from shot to shot.
Factual error: Near the end of the movie, Jamie Lee is speeding down a road in the coroner's van she has stolen and Michael is attacking her from the back of the van. She slams on the brakes, and he flies into the front windshield, which shatters into many pieces. Windshields are made of laminated glass, which contains a sheet of plastic, sandwiched between to sheets of glass. It does not shatter when struck.
Continuity mistake: There was supposed to be a loss of power inside the house in the beginning, but Jimmy opens the fridge and the light comes on.
Continuity mistake: When Laurie Strode goes back into the school to battle Michael with an axe, she strikes him and it sticks in his shoulder. In the next shot you can see it's only left flat on his shoulder.
Continuity mistake: In the montage scene with the newspaper articles and etc. there is a shot that says "Class of 78" and Laurie Strode - Laurie would have graduated in '79 if she was a senior during the first Halloween, since it was October of '78, and graduation would have been the next year.
Continuity mistake: At the end, Jamie Lee Curtis has two kitchen knives and continually stabs Michael until he falls off the balcony and onto a table. You can obviously see that it's a dummy because the dummy falls on its side if you look closely the right side of the table collapses, but then they switch to another shot with a real person and it now shows the left side of the table has collapsed.
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,.