Continuity mistake: In the scene where Laurie walks across the street to check on her friends, the porch of the Doyle house has nothing on it. When Laurie runs back across the street and starts pounding on the door she picks up a flower pot off the porch to throw at the window. (01:11:20 - 01:18:30)
Continuity mistake: When the girls are walking home from school, Annie says that she is babysitting Lindsey Wallace and Laurie says she is babysitting Tommy Doyle. Laurie also claims that the Wallace's and the Doyle's house are 3 doors down, but when Annie drops Laurie off the houses are right across the street from each other.
Continuity mistake: Annie's underwear changes patterns from a beige color to a floral pattern when she's stuck in the window.
Continuity mistake: When Laurie is in the bottom of the closet hiding from Michael, you can see that a purple dress is in front of the rest of the clothes. However when she stands up to grab a hanger, a pink sweater is in front only to be behind the purple dress again when she sits back down.
Continuity mistake: When Annie takes Lindsey round to watch TV with Tommy, they leave the TV in their house on. But when Annie comes back looking for her car keys, the TV is off.
Continuity mistake: After Laurie asks Annie what she is going to wear to the dance (while Annie is driving with Laurie), we see Annie turn the steering wheel to the left, yet the car turns right as seen on the green screen in front of them. As the car is turning right, it cuts back to Annie still turning the steering wheel left. (00:33:45)
Continuity mistake: Toward the end of the movie, Dr. Loomis comes up the stairs and starts to shoot Michael six times, sending him spiraling to the terrace. Take notice that Michael is about to go over the ledge of the terrace head first. In the next shot it shows him carefully backing off the ledge, while facing the house. (01:26:45)
Factual error: It's supposed to be late autumn, but in most scenes the leaves are still very green. [This is because it was shot in California during the summer. Carpenter imported leaves for every shot that you saw leaves blowing on the ground and blew them with a giant fan (this also killed any ambient sound takes); there are also a few shots where you can see palm trees (most prominent the one right in the front yard of Michael Myers' house).]
Visible crew/equipment: During the opening sequence, the scene is shot in first-person point-of-view from the young Michael Myers. As he walks through the house, the shadow of the camera can be seen several times. After he picks up the knife in the kitchen, the camera's shadow is visible on the door frame leading into the dining room. It can be seen again just before he turns to go upstairs. (00:04:05)
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.
Continuity mistake: When Lynda is talking about how she forgot her books (at the beginning of their walk home) we see Michael Myers drive by, then speeds up and drives passed a driveway, then it cuts to Annie (Myers is still driving) and she says, "Hey Jerk. Speed Kills." We see Michael driving again and he is just a bit behind the drive-way he drove by before, he should be much farther ahead down the road at the speed he was going. (00:21:35)
Continuity mistake: The young girl walks up to the Myers house near the start. From the view in the house she reaches the door. Camera changes and she's halfway up the steps. (00:13:20)
Revealing mistake: In the scene outside of Smith's Grove mental institution, where Michael jumps on top of the station wagon, he attacks the nurse, but she manages to wrangle free. A few moments later, he smashes the passenger side window. If you look closely, the actor's arm has a wrench taped to it in order to effectively break the glass injury free. (00:10:30)
Revealing mistake: When Laurie is kneeling down talking to Tommy and Lindsey, you get a good view of her arm where Michael stabbed her. You can plainly see it's just a streak of stage blood with no cut at all.
Visible crew/equipment: Just after Michael has picked up the knife, a stage light can be seen in a silver pot. This is as he walks into the next room. (00:04:15)
Visible crew/equipment: At the end of the opening murder scene, when Michael's father takes Michael's mask off, you can see the shadow of the camera cast on Michael's father as it pulls away. (00:06:35)
Revealing mistake: When Annie is putting her clothes in the wash the door slams shut and she walks over to try and open it, look closely and you can see her turn the deadbolt and lock it so she can't open it. (00:46:45)
Continuity mistake: When Bob and Linda arrive at the Wallace house, the exterior shots show the doorknob on the front door to be on the left side which would mean the door would swing inward to the right. However, when the shot cuts to the inside of the house as the door opens, the door swings inward to the left and the doorknob is now on the right side. (00:59:20)
Continuity mistake: In the scene where Tommy Doyle looks out the window and sees the shape outside of the house across the street, the porch light is off and the jack o lantern is not lit. When he attempts to show Laurie, the next shot of the house shows that the porch light is on and the jack o lantern is lit.
Revealing mistake: When Michael stabs Judith, he uses a rather large butcher knife. Yet on her chest there's no "wounds." It just looks like smeared stage blood. He stabs her deeply as well. We see this when Michael goes outside to his parents. Blood is halfway up the knife.
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