Continuity mistake: In the scene where Laurie walks across the street to check on her friends, the porch of the Doyle house has nothing on it. When Laurie runs back across the street and starts pounding on the door she picks up a flower pot off the porch to throw at the window. (01:11:20 - 01:18:30)
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: After Laurie asks Annie what she is going to wear to the dance (while Annie is driving with Laurie), we see Annie turn the steering wheel to the left, yet the car turns right as seen on the green screen in front of them. As the car is turning right, it cuts back to Annie still turning the steering wheel left. (00:33:45)
Continuity mistake: Toward the end of the movie, Dr. Loomis comes up the stairs and starts to shoot Michael six times, sending him spiraling to the terrace. Take notice that Michael is about to go over the ledge of the terrace head first. In the next shot it shows him carefully backing off the ledge, while facing the house. (01:26:45)
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).]
Visible crew/equipment: During the opening sequence, the scene is shot in first-person point-of-view from the young Michael Myers. As he walks through the house, the shadow of the camera can be seen several times. After he picks up the knife in the kitchen, the camera's shadow is visible on the door frame leading into the dining room. It can be seen again just before he turns to go upstairs. (00:04:05)
Continuity mistake: When Lynda is talking about how she forgot her books (at the beginning of their walk home) we see Michael Myers drive by, then speeds up and drives passed a driveway, then it cuts to Annie (Myers is still driving) and she says, "Hey Jerk. Speed Kills." We see Michael driving again and he is just a bit behind the drive-way he drove by before, he should be much farther ahead down the road at the speed he was going. (00:21:35)
Revealing mistake: In the scene outside of Smith's Grove mental institution, where Michael jumps on top of the station wagon, he attacks the nurse, but she manages to wrangle free. A few moments later, he smashes the passenger side window. If you look closely, the actor's arm has a wrench taped to it in order to effectively break the glass injury free. (00:10:30)
Continuity mistake: When Bob and Linda arrive at the Wallace house, the exterior shots show the doorknob on the front door to be on the left side which would mean the door would swing inward to the right. However, when the shot cuts to the inside of the house as the door opens, the door swings inward to the left and the doorknob is now on the right side. (00:59:20)
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.
Continuity mistake: In the beginning, when the doctor is talking to the new nurse for Michael in the car, the nurse is smoking a cigarette, but seconds later she has just lit one up. If she smokes one in 45 seconds then lights up another she is one worst chain smoker I've ever seen. (00:08:05)
Plot hole: When Laurie and her friend see the Sheriff in front of the hardware store, he tells them someone broke in and stole "A Halloween mask, some rope, and a couple of knives." Without someone doing a complete inventory of the store, there would be no way to know the specific items that had been stolen, especially when they are such commonplace things as knives and rope in a hardware store.
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.