Knock Knock (2015)

Continuity mistake: When Keanu is talking with his wife on the tablet, he closes his shirt with his left hand, but in the image of him on the tablet, his shirt is still open.

Genesis: Knock, knock.
Bel: Who's there?
Genesis: Cheating Evan.
Bel: Cheating Evan who?
Genesis: Cheating Evan-tually gets you killed.

Question: How did Bel and Genesis know about the conversation Ethan had with his family before his family left for the beach? He asked them were they spying on him, but the conversation was in the house. How do they know what was said?