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.
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: 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.