When Dr. Loomis & Dr. Hoffman are in the mental hospital, Dr. Hoffman answers the phone. He says, "I see. I see, thank you," but his mouth movements don't match the words. We then hear him yell, "Loomis! Loomis!" but he is clearly not actually yelling.
The blue sedan Dr. Loomis parks at the pumps outside the gas station is different than the one that blows up minutes later in the same location during a fire. The vehicle he parks is a 1982-1983 Chevy Malibu with four headlamps and one kind of grille. The vehicle that explodes in the fire is a 1980-1981 Malibu with two headlamps and a different grille.
The burn scars on Dr. Loomis' face change periodically throughout the movie. There is a dramatic change from when he is riding in the truck with the preacher on his way to Haddonfield compared to when he arrives and comes into the police station.
When Jamie and Rachel get into the cop car and Sheriff Meeker drives off, you see Michael Myers standing in the road. Right before the scene ends, crew making the smoke blow are visible on the far left of the screen.
When Dr. Loomis is hitchhiking down the road and the preacher picks him up, mountains can be seen in the far distance of the direction that they're traveling. There are no mountains anywhere in the state of Illinois.
Michael Myers had been at Smith's Grove - Warren County Sanitarium prior to his escape in Halloween and was being transferred to that location at the beginning of this movie. The patches on the medical personnel say Smith's Grove, but the ambulance is labeled as Smith Grove.
Michael throws a man into the power grid, effectively killing all of the power in Haddonfield. Even though all of the houses are completely dark, there are inexplicable sources of light that are too bright to be the moon coming from behind trees and buildings, as well as shining in through windows from angles that would be impossible.