Continuity mistake: After Ben Tramer gets hit by the police car, his body can be seen and heard slumping. In the next shot, he is pinned fully upright.
Visible crew/equipment: The crash mat is visible when Michael Myers falls off the balcony in the beginning of the movie.
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).
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.
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.
Factual error: At the end when Loomis lights the lighter, the room explodes and is engulfed in flames. Loomis would've burned to death quickly yet shows up in part 4 with burns on his face and arm.
Suggested correction: First, surviving an explosion is unlikely, but not impossible - there are plenty of real-life stories of people surviving fiery blasts. Second, I would argue that it definitely falls firmly under suspension of disbelief, and therefore I don't believe it's a valid mistake. And third, even if it was a valid mistake, this should be a mistake under the "Halloween 4" page, not the "Halloween 2" page.
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).
Character mistake: When Ben Tramer's friends report him missing to the police, they describe him as being 17 wearing a mask. This is what leads Loomis to make a positive ID... on Halloween.
Character mistake: The coroner determines that the burnt body of Ben Tramer (still unidentified) must be young, based solely on the absence of fillings in his teeth. Nobody considers for a moment that it could be a 30-something with good dental hygiene.
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.
Audio problem: When Loomis shoots Michael in the opening here his gun sounds different from the previous movie.
Continuity mistake: When Michael's shown falling off the Wallace balcony in the opening scene it's on the front of the house not the back. Clearly this is a different house. (00:02:31)
