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: When Randy is jumpscared by the guy playing football (something the killer couldn't predict), in the aerial shot the football dude is standing on the crossroad that is fully in the shade instead of partially in the sun like before the cut. The two girls Randy harassed are also gone from their spot, which means in 5 seconds they packed their books and stuff they had laid out in front of them. (01:03:30)
Continuity mistake: When the cameraman says he's "one verb away from vacating these premises", there's a blonde with a green backpack walking left to right in the background. The same blonde appears two shots later behind Randy, who is further to the left than Joel. (01:01:45)
Continuity mistake: During the Top Gun scene, Derek steps on the table where Portia De Rossi and the other sorority elites are having lunch. You see her fetch from the table the Pepsi can, but then she holds it at a different height at the cut. (00:48:15)
Continuity mistake: When Sidney answers the phone at the sorority, look at her sheer top under the jacket; reaches almost the neck (you can backtrack the moment when it rose from its optimal fit; she put the jacket in the same take and that ended up pulling the underneath garment a bit). At the next cut, the top rests lower as it should. (00:39:20)
Continuity mistake: On the couch, Cici asks the friendly voice "Who are you calling for?" Notice the way she holds the phone, and how her middle finger is on top of the last two fingers. It's tucked back in the close-up ("What if I say goodbye?"). (00:30:25)
Continuity mistake: Mickey says in no uncertain terms what does he think of the sorority girls from Delta Lambda. There's a reaction shot to Hallie's "Hey!" that features just Timothy Olyphant and Jerry O'Connell, and Olyphant pulled the sleeves of his oversized T-shirt up quite a bit because it is 200% baggier at his shoulders. (00:22:10)
Continuity mistake: After Sidney backhanded Gale, Joel reacts with a weird falsetto (actor improv, according to the DVD commentary) mocking Hallie. He had his left hand off the equipment, but in that reaction shot he's holding it with both. (00:25:35)
Continuity mistake: The sorority girls that invite Sidney at the party when she is watching the press conference leave turning around in unison. The taller black girl in the background though handwaves in the first shot but in the second, when she is in front, her arm is all the way down. (00:22:00)
Continuity mistake: Randy brings Sidney's attention towards Gale at the press conference. When Randy says "Starring Gale Weathers", Jerry O'Connell is gripping the shoulder of his backpack, and he isn't in the shots before and after. (00:21:00)
Continuity mistake: Gale is spearheading the other reporters' walk towards the press conference after she just humiliated Debbie. One of her colleagues asks her what's she doing there and if she thinks the murders are connected. To the right you can see a young redheaded photographer in both the shots when the question takes place, but in the first she's walking slowly and fixing the camera, in the second she cuts in resolutely and perfectly prepared. (00:20:15)
Continuity mistake: When the Artsy Teacher (that's how he's credited) asks if the student thinks the killer is trying to make a real life sequel, he's sitting straight. He is then suddenly hunched over in the next shot. (00:16:00)
Continuity mistake: During Maureen's death scene, at one point the killer has an arm around her waist and as they make their way through the crowd of the most unruly moviegoers in history there's a girl in a blue dress that runs past them. Behind her there's a blonde lady that is drinking from a Pepsi paper cup and stands to cheer. In the next shot she's still cheering but from her seat, her cup switching hands too. Subsequent shots from the initial angle feature her again bopping on her feet. (00:10:45)
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: 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)
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: 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 Gale watches Dewey agonize behind the glass, her hand is in different positions and spots between shots. (01:22:15)
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: 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)
Answer: His main motive was fame, or infamy for that matter.