Other mistake: At the start of the film, Michael was 6 years old. It's exactly 15 years later when he gets out on Halloween night again which would make him 21. However the ending credits state the two Michaels as 6 and 23. (01:29:00)
Continuity mistake: When Annie and Laurie leave the hardware store to go to their babysitting jobs it is broad daylight. When they reach the houses apparently just a few blocks away, it is already dark.
Continuity mistake: During the beginning of the movie, when Laurie is talking to the police officer, the sun is shining bright as if it is a perfect day, then when Laurie walks to her house, the trees look as if it has just quit raining.
Factual error: There's no way that the knife that killed Bob was long enough to go through his body and impale the wall behind him. And a butcher knife cannot "pin" a human body to a wall and just stay put, they aren't strong enough.
Revealing mistake: When Tommy is carrying the pumpkin from school, you can see it has a cut in it to make it crack open when he falls down.
Plot hole: We see Michael wearing the mask throughout the day, from morning at the Myers House, to mid-day outside Laurie's school, to afternoon when Tommy is leaving school, yet when Laurie and Annie see the sheriff in front of the hardware store on the way to their babysitting jobs (presumably in late afternoon or early evening), the store's alarm is still ringing and the sheriff mentions that one of the items stolen was a Halloween mask.
Continuity mistake: In the scene when Michael is carrying Annie's body into the house her head is on the left side of his body. After they show Tommy looking out the window they cut back to Michael and suddenly Annie's head is on the right side of his body. (00:55:14)
Other mistake: The date on the tombstone above Annie says that Judith Myers was born November 10, 1947, and died October 31, 1963. That would make her 15 (not quite 16 yet) at the time of death, yet everyone says that she was 17 years old.
Revealing mistake: Even though the story is set in Haddonfield IL, all of the cars have California license plates.
Continuity mistake: When Laurie is walking home the sidewalk instantly goes from wet to dry. (00:25:25)
Visible crew/equipment: There is a scene where Michael Myers is hiding behind a hedge bush and Laurie and her friend go to investigate. Laurie turns around and talks and you see cigarette smoke go by the camera (John Carpenter admitted on the commentary track that he was smoking a little too close to the camera that day). (00:24:10)
Visible crew/equipment: Just before Laurie stabs Michael with the knitting needle you can see that one of the window shutters is open and that the wind is ruffling the curtain. When the camera gets closer and the curtain is out of the way you can see the huge fan in the studio being used to simulate the wind outside. (01:19:50)
Factual error: When Dr. Loomis is at the pay phone talking to someone at the mental institution, the background of the scene has mountains, which in fact Illinois has hardly any mountains, and the few it does have are nowhere near there. (00:18:35)
Continuity mistake: In the beginning of the movie, Laurie runs into Tommy while she is walking to school, meaning that they live near each other; however, later on in the movie, when Laurie has to babysit Tommy, Annie has to drive her to Tommy's house, which is located on the other side of town.
Suggested correction: The film never says Tommy lives on the other side of town. They were only driving so long as an excuse to cruise around, smoke weed and kill time. Loomis later tells Sherriff Brackett that he's walked 3 blocks from the Myers house and soon after that we see him arrive at the Doyle house, still on foot. Since we know the Strode and Myers houses are within walking distance of each other, we also know the Doyle and Wallaces aren't too much further away.
Earlier on the walk home Annie says the Wallaces are leaving at 7. She later calls Laurie and says she will pick her up at 6:30. Using those times, the ride couldn't have taken for than 30 minutes, part of which was occupied by the stop at the hardware store (when it's still daylight). There's really no way to reconcile this timeline.
Continuity mistake: When Laurie is beating on the door to Tommy's house and telling him to open it, the door knob is on the right side of the door. When it shows Tommy opening it from the inside, the door knob is also on the right side (it should be on the left). (01:18:35)
Visible crew/equipment: When Annie is locked in the laundry room, there's a scene where the phone is ringing incessantly and the camera pans slowly to the right showing little Lindsay Wallace who's too busy watching television to answer the phone. If you watch this scene in widescreen, you'll see a crew member lurking in the shadows (left side of the screen) on the wall. It appears they are trying to avoid being seen. (00:47:10)
Other mistake: Immediately after the scene where Laurie is talking to Linda while she is being strangled by Michael, Laurie looks out the window and sees the LOWER level of the other house lights turned on. Michael was on the upper level and could not have gotten down the stairs to the first level in time (1 or 2 seconds) to have turn on the lights. (01:08:20)
Continuity mistake: When Laurie, Annie, and Lynda are walking home from school, the sidewalks are wet. When Laurie is by herself, they are dry.
Continuity mistake: When Annie drops Laurie off at Tommy Doyle's house, when she walks in their front door it has multiple glass panels, but later it is a solid, plain white door. (00:34:45 - 00:39:55)
Continuity mistake: When Laurie gets into Annie's car, she throws her coat in the back. During that shot when both of the girls are visible, the rear view mirror on the windshield between the two of them is level. When it cuts to only Annie and Laurie in the subsequent shots, the rear view mirror is crooked, tilting to one side.
