Factual error: Federal institutions have strict rules for inmates or patients based on the reason for their incarceration, including substituting metal utensils with safer plastic ones. Despite being convicted of brutally killing his family, Myers is still trusted with a sharp metal fork - resulting in the nurse's death.
Revealing mistake: In the beginning of the movie, when Michael removes the knife from the drawer, you can see the blade bend.
Other mistake: In the original ending, after Laurie is pulled out of the cop car, Loomis is able to reason with Michael to let her go. After he does, he is shot to death by the police. This was obviously changed, but when Loomis is reasoning with him in the final cut, you can still see the police standing behind him. (01:46:00)
Plot hole: Laurie calls the police from 1987 Winchester drive, and gives them that address. Yet the police responding to the call go directly to 1960 Winchester drive - where Laurie has run to and is now hiding - even though it's on the other side of the street and several houses down. (01:31:30 - 01:33:40)
Continuity mistake: When Laurie's mom is struggling to get to the phone after Michael stabs her, she reaches for it, knocking it off the hook. When Sheriff Brackett calls later on, the answering machine picks up.
Continuity mistake: Laurie drops the items in the mail slot twice at the Myers' house. (00:58:30)
Continuity mistake: In the beginning, after Michael kills his sisters boyfriend, he puts the mask on with bloody hands yet he gets no blood on the mask.
Continuity mistake: Laurie's hair changes from curly to straight frequently in the Myers' house.
Continuity mistake: Near the film's end, Myers crushes Dr. Loomis' skull with blood protruding from several parts of his face. In the following shot of him being dragged into the house dead, his face and head are perfectly fine.
Factual error: When Laurie, Annie, and Lynda are walking home from school on Oct 31st, you see the "fall" leaves on the ground, however the surrounding trees are as green as can be. (01:04:15 - 01:05:30)
Continuity mistake: When Laurie stops at the Myers house she has no backpack. She does in the next scene.
Continuity mistake: When Grizzly the truck driver stops at the truck stop, the attendant opens the hood and gets the hose twice.
Revealing mistake: In the rape scene, the guard hits Michael in the back with a baton. It leaves an indent in the material of the suit he's wearing underneath his robe.
Factual error: There's no way a body can be pinned to the wall with the tip of a butcher's knife.
Revealing mistake: When Michael is young, at Christmas, Dr. Loomis approaches Smiths Grove. There is snow on the ground, but the leaves on trees are green.
Character mistake: The title card shown after Deborah's suicide states that Michael's escape from the asylum takes place "Fifteen Years Later." Yet the night after the escape, Sheriff Brackett refers to her suicide having been seventeen years ago. (00:38:40 - 01:29:00)
Continuity mistake: When Michael beats the kid in a park with a stick, the attack only lasts a minute yet the new blood on the kids face is dried up, presumably from earlier takes. The dry blood effect is seen in more scenes throughout the movie.