Revealing mistake: Just before Charlie is stabbed you can see the "blood pack" under his shirt.
Revealing mistake: When Kirby is on the phone with Jill, as she approaches the "Stab fest" party, her iPhone is on the contacts list and not on call mode. iPhone screens are blank during phone calls when up against the ear. (00:54:10)
Revealing mistake: Sidney rubs her hand down the door frame after Olivia is killed, yet the blood doesn't smear. (00:36:30)
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)
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.