Continuity mistake: At the police station, Cotton sarcastically says "Ok then, book me." Look at the door the chief of police is walking towards, which has his name on. In close-up there is a string touching the C of "Captain", that is missing in the wide angle. (01:10:10)
Audio problem: When Gail and Dewy are watching the tape of their conversation in the park, the sound of their conversation is way too clear taking into account their distance from the camera and the microphone, and there is no ambient noise at all. (01:14:15)
Continuity mistake: Once Gale tells Dewey "I feel bad, Dewey. I feel really bad!" and it cuts back to the previous camera angle, the sunlight behind her (where Joel's taxi is and then was) changes abruptly. (01:14:25)
Continuity mistake: Outside the police station, Dewey stops because Gale is being sympathetic with her 'feel bad' speech. His hand is on the corner of the public phone in one shot, to the left in a different spot in the next shot. (01:14:35)
Continuity mistake: Dewey removes a VHS from the VCR. The VHS has a black spine. A minute later they begin getting jiggy with it and in the shot from the front of the desk (the rear view of Mrs. Cox, if you prefer) you can see on the VCR a white label on the spine. (01:17:10)
Audio problem: When Dewey is being stabbed up against the soundproof glass, you see a shot of Gale shouting "No." You can see a reflection of the killer in the glass standing motionless, but you can hear Dewey being stabbed. (01:18:45)
Audio problem: The killer replays for Gale and Dewey Randy's death. The words he uses are the same, but the pacing of his last line is off, and so is the part before; in the original scene Ghostface finished the "You'll never get the girl" line just as Randy touched the phoning bystander, but in this footage the line ends earlier than that moment. (01:18:45)
Continuity mistake: When Dewey and Gale are in the classroom and as they are making out, when they realise that they are being filmed, Dewey has one side of his jacket off below his shoulder and in the other camera angle it shows that he has it perfectly on. (01:18:50)
Continuity mistake: Courteney Cox's right hand is alternatively on the horizontal shelf or the vertical bar of the archive when Dewey tries to warn her from the other side of the glass. (01:21:45)
Continuity mistake: When Dewey is trying to get Gail's attention on the glass, his hand changes from being open palmed to a fist, between shots. (01:21:50)
Continuity mistake: The amount of blood on Dewey's forehead at the very beginning of the scene of his 'death' changes between shots. (01:21:55)
Continuity mistake: When Gale watches Dewey agonize behind the glass, her hand is in different positions and spots between shots. (01:22:15)
Continuity mistake: In the sound room murder, Gale stares at Ghostface from the other side of two different bloodstains on the glass. (01:22:30)
Continuity mistake: When the killer attacks Sidney and the bodyguards in the car, he smashes through the driver's side window, leaving some of the glass still around the edges. When the killer drives off, these fragments have dissapeared and the window is totally gone. (01:22:40)
Revealing mistake: Just before the cop gets impaled through the head by the pole, you can see that it is obviously a dummy. (01:23:45)
Continuity mistake: At the dorm, Sid and Allie hop in the cop car with Sid in the left backseat and Allie in the right backseat. In the following scene they have swapped. (01:24:25 - 01:25:50)
Continuity mistake: When Sid and Allie start freeing themselves from the wrecked police car, the metal pipe left a tiny hole in the grid, which is immediately larger in the next shot. (01:28:15)
Revealing mistake: When the killer kills Hallie you can plainly see the stuntman's eyes through the mask eyeholes. (01:32:00)
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)
Answer: His main motive was fame, or infamy for that matter.