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.
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.
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 is at Tommy's school, there are cars parked across the street. When he gets in the car to leave, they are gone.
Continuity mistake: Laurie's hair keeps changing throughout the movie. Her hair changes from wavy to straight while she babysits the kids. It is wavy again when she is going to Lindsey's house, and straight again when she is running back to Tommy's house. It stays straight until Michael starts to strangle her,where it becomes wavy again, having been straight in the previous shot. It all ends with straight hair. Surely all of this could not happen in a matter of hours.
Revealing mistake: In the scene where Dr. Sam Loomis is calling from a pay phone warning of Michael Myers return to Haddonfield, a Southern Pacific freight train passes by blowing its horn (you can't see the train in the standard version). While SP trains are quite common in Southern California where the film was made, they don't actually travel through any part of Illinois. (00:19:15)
Deliberate mistake: The opening camera scene, that's supposed to be shot from Michael's POV as a child, is too high as noted that he's almost face to face with his parents at the end of the scene and then is only mid-chest up to the father when he removes the mask. The POV is also too high as he walks across the dining room and as he approaches his sister upstairs.
Continuity mistake: Michael breaks the passenger front door window on the wagon when he takes car, a short time later when he's driving in town the window is not broken.