Revealing mistake: When Loomis shoots the marshal's window out, when he makes a u-turn you can see the tire marks on the road and dirt from previous takes.
Continuity mistake: When Michael enters the Elrods' home and steals the knife, as he picks up the knife from where Mrs. Elrod had left it, there are several drops of blood that fall onto the lunchmeat, presumably from Michael's injuries from having been shot. A few seconds later when Mrs. Elrod walks back over to the lunchmeat and reaches for the knife and sees the blood, the droplet patterns are different.
Other mistake: Watch the scene where the drunk Ben Tramer (wearing a similar Michael Myers mask) was hit by the speeding cop car and then pinned against the parked van before burning. Even taking out the equation of accidentally hitting Ben Tramer, what was the cop car doing speeding at that rate of speed, head on towards a parked van on a side street? Remember, we only hear screeching brakes after the car hits Tramer, and we never see the cop car swerve or any of that to miss hitting the van. Seems that even if Tramer hadn't been there, the cop car was sure to barge head on right into that van.
Continuity mistake: At the end of the first Halloween, Loomis fires six audible shots from his gun (whether or not they all hit Michael is irrelevant). However, at the beginning of Halloween II, Loomis' gun can be heard firing off seven shots. Also the front view of the house from the first movie and second movie are two different houses. When Michael is shot he falls off the balcony which is on the side or rear. The second movie shows him falling off the front of the house. Different railings too.
Revealing mistake: In the beginning of Halloween 2, the scene where Dr. Loomis shoots Michael Myers repeatedly until he falls off the balcony is reenacted to reflect the events of the original Halloween. If you look carefully, you can see that the stuntman portraying Michael is actually walking backwards on a table until he haplessly falls off the balcony.
Revealing mistake: In the scene where Marion is calling for help on the police car's 2-way radio, she speaks, "Marion Chambers with Doctor Loomis at the clinic; He's here." Right after that, the policeman on the other end of the radio starts to talk. However, Marion never lets go of the "talk" button on the radio's microphone, until the middle of the policeman's response to her request. You can't listen and receive anything on the radio while the button on the mic is pressed.
Other mistake: In the credits, Michael Myers is said to be 23. However, the fact that he is supposed to be 21 in both films (because #2 took place directly after the first) is stated in both movies.
Visible crew/equipment: On the GoodTimes DVD release of Halloween 2, just after Dr. Loomis blows up the hospital room the camera switches to the hallway where Laurie is running away. The explosion comes through the hallway and you can see the right side of the hallway shaking - this shows that the wall is a studio set wall. You can also see wires behind it and the wooden supports that are holding it up. (01:25:30)
Continuity mistake: During the scene where Jimmy the paramedic finds Ms. Alves in the major surgery room on a stretcher with the blood completely drained from her body, while turning to walk out, he slips and falls back first in the pool of blood. Later, he finds his way to his car where Laurie had been hiding and attempts to get help. If you look closely, he still has the same shirt on, but it has no blood on it.
Continuity mistake: When Ben Tramer gets hit by the police car and trapped in the explosion, you can see and hear his body slump forward onto the hood of the car. In the next shot, he is pinned fully upright.
Revealing mistake: When Annie's body gets wheeled out of the house and her father closes her eyes, her lids blink even though she should be dead.
Continuity mistake: Jimmy walks across a huge puddle of blood before finding Mrs Alves dead on the stretcher. When he slips and is unconscious, you can't see any footprints, not even near the door where he came in.
Character mistake: The police claim the Myers house is at "45 Lampkin Lane." The address on the front porch clearly displays the number 709.
Continuity mistake: Dr. Loomis shoots out the passenger window of the marshal's car, but it seems to be intact in later scenes, like when the woman is outside using the radio calling for help.
Revealing mistake: When Janet spins the dead Dr. Mixter's chair around, you can see his right hand flinch.
Continuity mistake: At the very end of the first Halloween, Loomis' first shot hits Michael while he is struggling with Laurie. A few seconds later, he is staring down Loomis and breathing very heavily. At the beginning of Halloween II, the scene is reenacted to reflect the ending of the first movie, but here you cannot hear Michael breathing at all after being struck by Loomis' first shot.
Continuity mistake: After Jimmy slips in Ms. Alves' blood, his position changes between the side camera shot and the above camera shot.
Audio problem: When Michael bursts through the door at the hospital, Loomis fires his gun and Michael stabs him. He drops his gun on the floor and it makes no sound when it hits.
Factual error: At the end of the film where the blinded Michael is swinging his scalpel aimlessly, it makes loud "swooshing" sounds. This is pure fantasy. I have personally mimicked what Michael did using similar small scalpels and even larger knives without almost no sound when swinging them.
Continuity mistake: When Annie's dead body is rolled out on the stretcher her dad pulls the sheet back and revealing her slit throat. After a brief cut to him, when it cuts back to Annie the sheet covers her throat.