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.
Revealing mistake: When Doctor Loomis touches the blood in the beginning, you can see a bit of material (it appears to be plastic) sticking to his fingers as he pulls them up from the grass. It appears as though it was somehow involved in the effect of him touching the blood. (Perhaps to keep it from soaking into the ground?).
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).