Halloween - Smoke
this occurs in the scene where annie and linda are walking home from school and see michael myers hide behind the bush. as soon as annie approaches the bush and looks behind it watch very closely as the camera cuts to annie, you can see a puff of smoke go across the screne. probably a crew member smoking too close to the set would be my guess. it is kind of trivial and probably not even worth reshooting, but it is definently there.