In the opening credit sequence, when the various photos/news clippings are being shown on the wall, there is a picture of Laurie Strode's graduation day.

On it, it says "Class of 1978". This is clearly wrong, because in the film Halloween, the month is obviously October, and we are told Laurie is in her senior year. Which means she won't graduate until summer in the next year, i.e. 1979.