Revealing mistake: In the beginning when Michael crawls out of the mine shaft that was just bombed he floats downstream. You see that he floats up to the old man's dock. If you look at him you can clearly see that he is wearing a black lifejacket. The filmmakers didn't even try to hide it under his suit, it is clearly visible on the outside of his jumpsuit.
Audio problem: When Tina is in the car with who she thinks is her boyfriend but is really Michael Myers; watch her lips when she says the line "Nothing Mike, that's what I get, Nothing." It is obvious that this line was dubbed in over something else as her lips do not match that particular line.
Continuity mistake: When Michael rams the pitchfork through Spitz, it is at the lower end of his back, nearer to his waist even. A few seconds later when Spitz's girlfriend comes at Michael with the same pitchfork, some other shots of the now dead Spitz show the pitchfork holes up near his shoulder blades, probably 6-8 inches higher than where they should be.