Continuity mistake: In the scene where Jill and Charlie are revealed as the killers, Jill is holding a knife to Sidney's throat and a gun in her left hand as she explains her motive. When Jill and Charlie are getting ready to stab each other, the shot changes and Charlie hands Jill the knife, even though she just had it in her hand pointing it at Sidney. (01:11:20)
Continuity mistake: When Sidney is in Olivia's room right after she killed, Sidney is holding onto the very bloody door frame, getting a little on her hands. The next shot when she picks up the phone, seconds later, there is no blood to be seen on her. (00:36:30)
Continuity mistake: At the Stab-a-thon, the original music in scream 2's showing of "Stab" uses "Red Right Hand" as the opening credits. The burning popcorn shot happens a lot later, and there is a window crash sound early in the scene that shouldn't happen until later. When 'Casey' is asked by the killer "are you expecting someone?", that's when she looks at the popcorn (which hasn't burned yet), and then she says "no".
Continuity mistake: Right before Sidney's aunt Kate comes back with her shopping, there is a yellow teapot on top of a chopping board on the kitchen counter in front of Sidney. She opens the door to find Kate. But when she comes back into the kitchen, the yellow teapot has suddenly moved about a foot away from the chopping board. (01:05:30 - 01:06:40)
Continuity mistake: When Jill stabs Charlie, the knife seems to make impact with his chest (directly in the middle). But then the close up he's bleeding from the heart (the right side of his chest, as we look at it).
Continuity mistake: As Charlie and Robbie are explaining the rules, Rear Window is written on the board. In the closer angle Charlie is standing to the right (viewer's left) of it, then from the wider angle Charlie has suddenly moved to the left so the writing is to his right. There is not enough time for Charlie to move.
Continuity mistake: When Olivia arrives home at night, she rings Kirby as she gets out of her car and ends the call as she is unlocking her front door. Kirby and Jill have a very short conversation, and then Jill prank calls Olivia. In the space of less than five seconds, Olivia has gotten into her house, gone upstairs, gone into her bedroom, turned on the lights, and is halfway through changing her clothes by the time Jill rings her. (00:32:30)
Continuity mistake: When Sidney and Jill are fighting, Sidney causes Jill's left eye to be bloodshot. But later its her right eye.
Continuity mistake: The phone in Jenny's hand disappears when she moves downstairs looking for Marnie. (00:07:55)
Continuity mistake: After Chloe stabs Rachel in "Stab 7", blood runs down her chin but in the next shot, her chin is clear of blood. (00:05:15)
Answer: Remember that Jill and Charlie were the killers and were working together. Therefore, Jill probably told Charlie the film they were planning on watching. Meaning when he called Kirby he knew what she and Jill were watching, which makes the killer more sinister and omnipresent.