Other mistake: At the hospital Laurie could not know that Myers was after her in there.
Other mistake: After Myers stabs the blonde nurse the scalpel is clean of blood.
Other mistake: Myers gets shot 6 times in the chest and body but has no blood or bullet holes.
Continuity mistake: At the beginning of the movie, the front door lights on the Doyle house are on, then off, then on again in just a few different shots.






Answer: There is a scene where Laurie dreams about meeting Michael as a young teen. It's unknown whether this is an actual memory of real events, but since nothing indicates otherwise, we could assume the he saw her at an older age when she looked closer to her 17-year-old self.