Factual error: Dewey wears a 6 point star badge but on his sleeve its a 7 point star patch. In any sheriff department its one or the other never both.
Continuity mistake: When Sidney and Jill are fighting, Sidney causes Jill's left eye to be bloodshot. But later its her right eye.
Audio problem: When Jill reveals herself as the second killer, we hear Sidney say "Jill". But her mouth doesn't move.
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.