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.
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.
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: 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.
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: When Michael emerges on fire from the blown up room, if you look closely, you can see plain as day the moment when they light the stunt man on fire for the stunt. (Suddenly you see fire erupt in the shape of Michael's body.) And if you look VERY closely, shortly before this you can even briefly kind of see the stuntman playing Michael in the doorway waiting to be lit on fire. (The bottom half of his body is visible for a split-second a moment before they light him up).
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.
Continuity mistake: When Laurie runs away from Michael Myers in the boiler room, she climbs a window and falls into a storage room. Her hospital gown gets flipped as she falls, exposing her underwear. The close-up shows her falling again, this time without the upskirt shot.
Plot hole: Early on, the TV in the Elrods' house is showing a report (corroborated by the authorities) of three bodies being found in the Wallace house. However, later on during the Ben Tramer accident scene, the deputy rushes up to the sheriff to inform him that three bodies were found (including his daughter Annie).