Character mistake: As we know throughout this movie series, Michael's middle name is Audrey. However when the doctor is typing Michael's release form, he types his middle initial as "M".
Suggested correction: Michael's middle name is only mentioned in an additional scene that was added to the television version of the original Halloween. If going by the theatrical versions Michael's middle name was never known, so not really a mistake.
Doctors at a sanatorium would certainly know Michael's middle name. It's been on file for decades.
Plot hole: At the end of Halloween 2 Michael is completely engulfed in fire for several minutes. Even if somehow he was to survive he would be covered in scars. When we see him at the beginning of part 4 he has a few small scars on his arms. Same with Loomis, he has a few small scars on his arms and one on his cheek despite being in a gigantic explosion and fire. Obviously this was done so they could bring the 2 characters back but it still makes no logical sense.
Suggested correction: Michael has the supernatural ability to heal as shown when he's shot in the eyes at the end of Halloween 2 because in this one he can see, as for Loomis it's possible he was blown into another room or into an area into the room where the flames couldn't reach him, no mistake.
Michael does have some scars though. If the excuse that he heals is the reason he doesn't have more than all of them would be healed after 10 years. Also Loomis was in the same room with Michael, even if Loomis was blown into another room he would have been engulfed in flames and would have been either dead from the shock wave or still burned worse than he is in part 4.
Suggested correction: This one's tough. On the one hand, this is intentional and if it were released today I'd chalk it up to being a (very) soft continuity reboot and that in this film the explosion was simply not as severe as it was in Halloween II. On the other hand, in the context of its release era, and just by watching you can sort of tell they were hoping viewers either had a fuzzy memory of Halloween II or were willing to just go with it. I think I'll err on the side of the former, but it's interesting.
The original script contained an opening scene which showed how Dr Loomis was blown out of the hospital. They had no enough money, so that scene was never shot.
Continuity mistake: When the sheriff's daughter is killed she goes from wearing a T-shirt to a flannel shirt.
Suggested correction: She changed her clothes because Rachel poured coffee on her T-shirt in a previous scene.
