This happens when Lily gets lost at the art show. It happens abruptly and you have to look closely. She's looking for a way out of the place when she gets shot with an arrow. She grabs it and turns around to look at her assailant. When they show "Valentine", look closely at his mask below his right nostril when he raises his arm to shoot her again. He already has blood on his mask. It shows her getting shot again, and then cuts back to his face. Look at his nostril again. There's no blood, but all of a sudden the blood starts trickling out of his nose. It helps a little if you can zoom in, but a good eye and a pause will help you.