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)
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)
Revealing mistake: When Tommy is trying to show Laurie that Michael is outside, something to the left of the Wallace house looks like legs moving around in the dark and then jumping up. Presumably, this is Nancy Kynes jumping up into Tony Moran's arms so that he can carry her around the front of the house, which is actually the shot that is used about ten minutes later. Carpenter potentially used this shot of the house either unaware that they could be seen, or didn't anticipate the future change in aspect ratio. (00:45:30 - 00:55:30)
Revealing mistake: When Annie is putting her clothes in the wash the door slams shut and she walks over to try and open it, look closely and you can see her turn the deadbolt and lock it so she can't open it. (00:46:45)
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.
Revealing mistake: Even though the story is set in Haddonfield IL, all of the cars have California license plates.
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.
Revealing mistake: Towards the end, Laurie pulls off Michael's mask, revealing his distorted face, but as he pulls the mask back down, the distorted face is also a mask as you can see the seam around his neckline.
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.
Revealing mistake: Lynda calls Laurie using a rotary phone yet doesn't even look at what she's dialing. And you can tell she's not actually dialing in the first place.
Revealing mistake: When Michael cuts Annie's throat, notice there's no visible cut or blood at all. When your throat is cut blood is going to pour out.
Suggested correction: True, but the view we have is distorted because it's shown through the windshield in a dark garage. Blood most likely poured out, but we don't see it because of how it's shown to us.
In the documentary, John Carpenter specifically stated that he left out of the gore on purpose. He wanted to film the movie so that the viewer would have a "mental image" of the kills without all the blood. He wanted it to be left to the imagination of the viewer.
