Continuity mistake: When Mickey shoots Derek, his bandage touches the ropes, but both before the gun fires and when Sidney cries out and steps closer, there are a few inches of distance between bandage and rope. (01:37:10)
Continuity mistake: When Gale is in the theatre and gets shot and falls from the stage, she has blood stain on her left breast (above the place where the shot wound appears). Yet later, when she climbs back to the stage, she has no blood stain, only the shot wound. (01:37:20 - 01:45:45)
Continuity mistake: When Derek dies after being shot, tied up on the stage prop, his head slumps down with it looking down to his right towards Sid with his forehead tilted to the left. When it cuts to another angle at a close up of Sid, suddenly his head is tilted the other direction. (01:37:40)
Continuity mistake: Mickey and Sid at the beginning of their struggle end up facing each other with a Greek pillar between them. The position of their hands changes between shots, literally touching at some point depending on if Olyphant's hand is higher or lower. (01:39:25)
Continuity mistake: When Mickey grabs the gun after the 'pillar fight', he holds it straight in the first shot, gangsta-style sideways in the next. (01:39:35)
Continuity mistake: When Sidney incredulously asks "Mrs...Loomis?" her left hand is raised at shoulder level, but her arm is straight in the next shot. (01:40:20)
Continuity mistake: When Debbie Salt is holding the knife on Sidney, the knife changes positions from time to time. (01:42:30)
Revealing mistake: When Mickey is flying through the air after getting shot at the end of the movie, if you look closely you can see it's a stunt double. (01:46:00)
Continuity mistake: During the struggle between Debbie and Sidney, in the phase that involves the fake stone block Sidney's hands have a different position on the block between shots, and so does the knife, stuck in it in different spots. (01:46:10)
Revealing mistake: When Mrs. Loomis dislodges the knife from the stone and lunges towards Sidney, the person falling back is not Neve Campbell, who comes back for the close-up shots. (01:46:15)
Continuity mistake: When Sidney and Mrs. Loomis fight with the knife, the weapon is bloodstained in different ways depending on the camera angle. (01:46:20)
Continuity mistake: Cotton shoots the killer, who when lying down has a giant hole in their throat. It wasn't there when both the killer and Sidney were falling backwards. (01:48:40 - 01:49:05)
Factual error: At the end of the film there is an LA county sticker on the ambulance, but the events are nowhere near LA. (01:52:05)
Continuity mistake: When Sidney is about to get in the car with the policemen, she has a rather large noticeable spot on her chin. In the following scenes, it is gone.
Revealing mistake: When the killer attacks Sid's bodyguards, the younger one has blood on his throat before the killer slashes it.
Continuity mistake: When Gale kisses Dewey she has no more lipstick on her mouth, then in the next shot she has lipstick on.
Revealing mistake: When Mickey draws the gun at the end of the movie, a hammer is heard being pulled back. Yet the gun that Mickey is using is hammerless.
Plot hole: After Sid and her friend get out of the car, they run away from it. Yet somehow while they are talking, the killer climbs out of the car and ends up behind them without them seeing a thing.
Continuity mistake: A pimple is visible on Sidney's chin as she says goodbye to Derek before getting into her ride with the two detectives and Hallie. Minutes later, in the scenes inside the car, the pimple has completely disappeared, even in the very extreme closeups.
Continuity mistake: After the accident - when the car is seen from behind - you can't see the killer (it's before Sid crawls over him).
Answer: His main motive was fame, or infamy for that matter.