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.
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: 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, Annie, and Lynda are walking home from school, the sidewalks are wet. When Laurie is by herself, they are dry.
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.
Continuity mistake: When the girls are walking home from school, Annie says that she is babysitting Lindsey Wallace and Laurie says she is babysitting Tommy Doyle. Laurie also claims that the Wallace's and the Doyle's house are 3 doors down, but when Annie drops Laurie off the houses are right across the street from each other.
Continuity mistake: Annie's underwear changes patterns from a beige color to a floral pattern when she's stuck in the window.
Continuity mistake: When Laurie is in the bottom of the closet hiding from Michael, you can see that a purple dress is in front of the rest of the clothes. However when she stands up to grab a hanger, a pink sweater is in front only to be behind the purple dress again when she sits back down.
Continuity mistake: When Annie takes Lindsey round to watch TV with Tommy, they leave the TV in their house on. But when Annie comes back looking for her car keys, the TV is off.
Factual error: It's supposed to be late autumn, but in most scenes the leaves are still very green. [This is because it was shot in California during the summer. Carpenter imported leaves for every shot that you saw leaves blowing on the ground and blew them with a giant fan (this also killed any ambient sound takes); there are also a few shots where you can see palm trees (most prominent the one right in the front yard of Michael Myers' house).]
Other mistake: When Annie goes to get her car keys, notice she goes through the back yard and goes to the front of the house as if the garage is in the back yard, when we see the garage on the left side of the house.
Revealing mistake: When Laurie is kneeling down talking to Tommy and Lindsey, you get a good view of her arm where Michael stabbed her. You can plainly see it's just a streak of stage blood with no cut at all.
Continuity mistake: In the scene where Tommy Doyle looks out the window and sees the shape outside of the house across the street, the porch light is off and the jack o lantern is not lit. When he attempts to show Laurie, the next shot of the house shows that the porch light is on and the jack o lantern is lit.
Revealing mistake: When Michael stabs Judith, he uses a rather large butcher knife. Yet on her chest there's no "wounds." It just looks like smeared stage blood. He stabs her deeply as well. We see this when Michael goes outside to his parents. Blood is halfway up the knife.
Revealing mistake: During the scene when Michael steals the station wagon from Dr. Loomis and the nurse, when she is lying across the seat you can see the passenger window is clean and very much transparent. However in the very next scene where Michael goes to smack his hand on the window, it is now foggy - a side effect of the glass alternative used for the effect.
Visible crew/equipment: In the scene where Dr. Loomis makes a call to Haddonfield police in a phone booth by the train tracks, you can see a reflection of a crew member in the phone booth glass.
Continuity mistake: In the scene where Laurie comes downstairs after taking a shower, she is at her front door singing to herself and she is looking out a window directly to the left of the door. Then there is a shot of the front of her house outside and you see her exit through the door that had the window next to it, but there is no longer a window there.
Continuity mistake: When Michael stabs Laurie in her arm the wound doesn't bleed at all, and there's no blood on the knife either. Then when she kneels down to talk to the kids, and in every scene after that, there's a streak of blood.
Continuity mistake: When Michael is at Tommy's school, there are cars parked across the street. When he gets in the car to leave, they are gone.
