Continuity mistake: Towards the end of the film, when Dr. Loomis finally finds out Michael is at the hospital and orders the trooper's car to turn around. As the car turns around, the lighting outside appears to be pre-dawn however they arrive at hospital and it is still pitch black.
Factual error: When Michael corners Laurie in the elevator, the door shuts just in time and slams on his hand and just keeps closing, and Michael pulls away. Elevators have rubber guards that would cause the door to open again if someone's hand would get caught in the closing.
Continuity mistake: When Pamela Susan Shoop is being killed by Michael in the spa, the towel is tied arround her upper chest, then the towel falls down, revealing her breasts. In the next shot a split second later the towel is back up at he upper chest.
Revealing mistake: In the storyline, Michael Myers is supposed to only be 21 years old. Towards the end of the movie, right before Michael kills Karen in the hot tub, he puts his hand on Karen's shoulder. It is easy to tell just by looking at his hand that he would be much older than 21. (The actor who played Michael was Dick Warlock, who at that time was in his early 40's).
Continuity mistake: When Michael falls off the balcony, he falls into some green grass and a garden is next to him. Yet in the original "Halloween", Michael falls onto the ground with barely any grass around him, just a dirt patch and no garden in sight. (This is explained because they reshot the ending to "Halloween" 1.).
Continuity mistake: At the end of part one, Laurie's stab wound was just a streak of blood and none on her shirt at all. When they bring her out to the ambulance, there's now a lot of blood on her shirt and streaks of blood on her arm. (00:13:20)
Factual error: The story is supposed to take place on Halloween 1978, but some of the clothing and hairstyles have noticeable characteristics associated with the early 1980's; particularly those of Alice, Karen, and Darcy.
Continuity mistake: After getting out of the car, Laurie is on the ground beside Jimmy's car outside the hospital when she sees the Marshall's car pull up. When Loomis tells the Marshall he ought to handcuff him to the wheel, if you look, Jimmy's car is still visible in the distance, but Laurie is not in that shot.
Revealing mistake: When Myers hits the hospital guard on his head with the hammer from behind we can see the head of the mannequin they used in place of the guard.
Continuity mistake: In the scene where the boys ask Mr. Hunt about Ben Tramer, Mr. Hunt says that "it's just a little past 11." Following this up in the next scene Bud lures Karen into the room to kiss her and wish her Happy Halloween. During this scene, as Bud reaches up to kiss Karen, his watch is clearly visible and shows the time as 1:00.
Other mistake: When Jimmy finds Mrs Alves, he has to walk through the puddle of blood to get to her, but there are no footprints in the blood.
Continuity mistake: The flashing lights on the front of Bud and Jimmy's ambulance change places as they pull into the hospital driveway. They go from either the side of the hood to the front of the bumper.
Continuity mistake: Michael falls from the balcony in a different way from the first film.
Continuity mistake: Michael comes into Laurie's room and stabs at her bed, only to find pillows had been placed there. He then walks out and leaves the door open. When Jill returns from checking on a patient, the door is now closed and she opens it.
Factual error: The film takes place in Illinois on Halloween, October 31st, but you'd never know it by looking at the colors of the trees, they all have green leaves and there is almost no sign of Fall or ground leaves.
Continuity mistake: In the first scene when Loomis has just shot Michael. In first film, Loomis looks over the balcony and see that he is gone, but now he goes out and discovers that he is gone.
Continuity mistake: When Michael walks into the old couple's house, NIGHT OF THE LIVING DEAD is showing on the television. At that point, the film is just beginning, with Barbara and Johnny first pulling up to the cemetery. Much later in the film, the security guard is also watching NIGHT OF THE LIVING DEAD on television, and it's the scene where the cemetery zombie is attacking Barbara in her car. This is no more than ten minutes later in NOTLD, yet it's way more than ten minutes later in the evening of HALLOWEEN II.
Revealing mistake: Annie's dead body has a pulse on her neck.
Continuity mistake: At the end of the first film, Dr. Loomis looked down from the balcony to find Michael's body gone. However, in this one, he finds the body missing when looking out of the house's front door.
Factual error: The pagan religious festival Samhain is mispronounced: the correct pronunciation is more like "so-win." This is a character mistake on the part of Dr. Loomis, but as the movie makes him appear quite knowledgeable on the subject and gives it significant dramatic weight, it's arguably appropriate to attribute the error to production as well.
Answer: I have read that, in early drafts of the script, the hospital was a health clinic, not a standard hospital. This would possibly explain why there are only a small number of patients, though it doesn't explain why there is a maternity ward, or why the mother brings her son there for emergency treatment.