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.
Plot hole: Since this was to be the last Halloween movie, there is still a major plot hole with the way things went to make Halloween Resurrection. Yes, Michael switched outfits with the paramedic. In Resurrection, one of the nurses says, "Well, why didn't the paramedic say something?" She responds that his windpipe was crushed. Yes, this prevents him from speaking, but it doesn't prevent him from taking off the mask, proving it's not Michael.
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: 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: 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: 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.
Revealing mistake: Visible wires holding up Will when Michael holds him in the air.
Continuity mistake: When Jamie Lee Curtis hits Michael Myers with the van and they go over the cliff, you can see that the back doors of the van swing open, but when they get a frontal of Jamie going down the cliff in the van, the doors are shut, this happens about 3 times before the van finally stops.
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.