Character mistake: During the scene in which they are all arguing whether or not film sequels are better than the original or not, someone says Sigourney Weaver's classic line from the movie Aliens: "Get away from her you bitch", and Randy makes a smart comeback and tries to correct him by saying "I think the correct line was 'stay away from her you bitch'. This is a film class, right?" Wrong - the first guy was right. This line is truly unconvincing for the Randy character, being a big movie buff and all. Jamie Kennedy decided to improvise as Joshua Jackson actually made the initial mistake by saying the wrong line, causing a different mistake. (00:16:25)
Character mistake: Dewey says that it does not make sense that the killer, who at this point is characterized as a copycat of the Woodsboro team, attacked Sidney. Actually that's exactly what happened in the first Scream; after Casey and her boyfriend, Ghostface tried to kill Sidney. Dewey should know since he was the first responder on that call doing a puppet show at the door, and it's not only amazing that he wouldn't remember, but also that nobody corrects him and that they haven't run a timetable of the previous events at all. Of course in the end the pattern does not matter just as Dewey later says, but up to that point, the killer was following it to a T, so they are reaching the conclusion too early and against the evidence. (01:01:00)
Character mistake: Mrs. Loomis is crazy and does not really think things through (she finishes her speech saying "who gives a f..." and that she's untraceable anyway), however it's worth noting that when she tells Sidney the official version that the police will believe, she is wrong; she says it wiping the gun clean from prints and throwing it away, which means that the police would find the supposed murder weapon with neither Mickey nor Sidney's prints on it, and neither wears gloves. Moreover, she plans to disappear and she was prominently featured in the media coverage, so people would certainly investigate her at least as victim.
Suggested correction: I don't think that a psychopathic character acting irrationally and jumping to unlikely conclusions really constitutes a character mistake. But I do think it's also worth pointing out though that cops usually don't rely on fingerprints on guns anyway - the likelihood of finding a usable print on a gun is minuscule (only about 5%), and there's going to be traces of things like skin-oil and whatnot on it from being handled, so the cops will likely just assume it was used by someone in the room - the most likely candidate being either Sidney and Mickey. Mrs. Loomis is also using a false identity and has got surgery to change her face before, so she could likely disappear pretty easily. Real-life killers get away with disappearing all the time.
I am no expert in true crimes and forensics, I am just challenging the movie logic here (which is why I talk about the behaviour of a crazy character who is running exposition). What I get from your objection though is that the cops wouldn't be able to tell that she wiped the gun clean from prints and so that wouldn't stick out as suspicious? She didn't really change her face, since Sidney recognizes her when she gets a good look at her. Rewatching the scene anyway it's very evident that she does not really care because she simply puts her faith in the cops not being able to track the fictitious Debbie Salt, so I would be happy with a correction here, I was interested in pointing out that the whole first part about wiping the prints and throwing the gun aside does not seem to logically follow up, I take note that according to your objection using 'real science' and forensics practice it might not even be that.
Character mistake: The library of Windsor College is awful at cataloguing, since Sidney searches the "Social Sciences" section and her screen is full of books about Quantum Physics. (01:06:35)
