A yearbook page with Laurie Strode's picture on it is shown toward the beginning of the film. Above her picture it says "Class of 1978." But if she were in school in October 1978, then she wouldn't normally have graduated until the following spring, in 1979. So therefore, she is Class of 1979.
When Norma (Janet Leigh) startles Laurie (Curtis) after the buses leave, she is as tall as Curtis (to give a proper scare) but in the next shot, from behind Laurie she is much shorter. Also, earlier in Laurie's office, Norma is clearly almost a foot shorter than Laurie/Keri.
The look of the mask changes back and forth for the scene where Charlie gets killed upstairs (the original mask was deemed unsatisfactory after principal photography was complete and some, but not all, scenes involving the mask were re-shot).
When Michael Myers crashes through the window with the knife in his chest the knife is sideways as if tucked under his jacket, when Laurie Strode/Keri Tate inspects the body in the next scene it is as it should be, vertical as if thrust into the body
As Keri/Laurie is walking down the sidewalk in town she briefly watches a group of trick or treaters walking across the street. This distraction causes her to bump into a group of trick or treaters on her side of the street. The group across the street and the group she bumps into are the same, judging by the costumes they are wearing.
When Laurie is driving the van with "Michael" in it, notice in the long shots the road is solid pavement or concrete, a very "beaten path," however inside the van if you look outside the back window you can see the road is in excellent condition and has the yellow stripe down the middle separating the lanes.
The goof items below may give away important plot points.
When the Van pins Michael Myers to the fence in the long shot just after the crash the engine bursts into flame. In the close up when Laurie Strode/Keri Tate is talking to Michael before beheading him there are no flames.