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: 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.).
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.
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.
Continuity mistake: Nurse Jill turns around before Michael Myers stabs her. After she's stabbed, she's already facing forward and her head doesn't snap to face the camera.
Continuity mistake: After Nurse Karen is drowned by Michael Myers, he drops her backward. The close-up shows however that she falls forward.
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).
Continuity mistake: The front door on the Doyles' house is different from the one seen in Halloween. You can see this when Dr. Loomis leaves the house in the beginning of the movie.