Revealing mistake: After Jill's mom returns home from the store, she pulls a box of cereal out of the grocery bag and it is already opened. (01:06:25)
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.