Continuity mistake: When Dr. Loomis is thrown through the window by Michael Myers in the school notice that Myers' hair has strangely turned white. The next shot Myers' hair has gone back to being brown. This is more evident in the widescreen version.
Plot hole: Near the end of the movie, Jamie and Rachel are being taken down the highway to safety by the vigilantes when Michael climbs into the back of the truck from being underneath it. Michael then struggles, stabs, and violently throws two of the vigilantes from the truck, and the third vigilante, standing only inches from the other two and still holding guard in the back of the truck, appears to see or hear none of the commotion. Impossible, even for horror films.
Audio problem: When the head of the Federal Sanitarium gives over control of Michael to the Doctors from Smith's Grove, he says, "I can safely say that Michael Myers is in your hands." If you watch his lips, you can see that the sound of the words and his lip movement is way out of sync.
Plot hole: At the end of Halloween 2 Michael is completely engulfed in fire for several minutes. Even if somehow he was to survive he would be covered in scars. When we see him at the beginning of part 4 he has a few small scars on his arms. Same with Loomis, he has a few small scars on his arms and and one on his cheek despite being in a gigantic explosion and fire. Obviously this was done so they could bring the 2 characters back but it still makes no logical sense.
Continuity mistake: Pay close attention to the scene at the end of the movie when Sheriff Meeker and several other troopers start shooting Michael. As the shots are being fired by multiple men, you will notice one of the troopers on the far left side of screen (blue/white flannel shirt, beard) lighting a stick of dynamite. As the shots pan back and forth between the troopers firing their shots and the shots hitting Michael, you will see this same trooper continue to keep lighting the dynamite. A few seconds later in the sequence while more shots are being fired at Michael, you will now see that same trooper now suddenly holding (and about to put down) a shotgun, the dynamite is now nowhere in sight.