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.
Revealing mistake: Even though the story is set in Haddonfield IL, all of the cars have California license plates.
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 to close to the camera that day).
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.
Continuity mistake: In the closing credits, Michael Myers is said to be 23 years of age, he was actually 21.
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).
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.
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 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.
Other mistake: At the beginning, when Michael is about to kill his sister, we see through his eyes. When his sister turns around to see him, she has to look way up, meaning that he would be very tall, when in fact, he is only 6 years old.
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.
Continuity mistake: The Myers' house is close enough to be walking distance from where Laurie lives. Yet at the end of the movie when Loomis notices the station wagon right down from the Myers' house, Loomis himself is a mere walking distance from the houses at which Laurie and Annie were babysitting. This is impossible since we were already told the street where the babysitting was taking place at was on other side of town from where Laurie lived.