Other mistake: In 2000 a Limited Edition of Halloween was released with 12 extra minutes of footage, filmed during the making of Halloween II in 1981. In order to air Halloween on TV they needed to remove and add certain scenes. In the original 1978 version, Dr. Loomis tells Sheriff Bracket, "I watched him for fifteen years, sitting in a room, staring at a wall, not seeing the wall, looking past the wall, looking at this night, inhumanly patient." In the Extended version, you'll see Dr. Loomis visit Michael in his room, and he's staring out of a window. Not a wall. (00:11:31 - 00:12:38)
Other mistake: Immediately after the scene where Laurie is talking to Linda while she is being strangled by Michael, Laurie looks out the window and sees the LOWER level of the other house lights turned on. Michael was on the upper level and could not have gotten down the stairs to the first level in time (1 or 2 seconds) to have turn on the lights. (01:08:20)
Other mistake: At the start of the film, Michael was 6 years old. It's exactly 15 years later when he gets out on Halloween night again which would make him 21. However the ending credits state the two Michaels as 6 and 23. (01:29:00)
Other mistake: The date on the tombstone above Annie says that Judith Myers was born November 10, 1947, and died October 31, 1963. That would make her 15 (not quite 16 yet) at the time of death, yet everyone says that she was 17 years old.
Other mistake: When Annie goes to get her car keys, notice she goes through the back yard and goes to the front of the house as if the garage is in the back yard, when we see the garage on the left side of the house.
Chosen answer: Because it was too graphic and elaborate for her to think it might be a prank. Not to mention that there was probably a very distinctive smell to the room.
JC Fernandez