The year I lived with my brother in college. He was in the bedroom playing WoW when I told him I had a girl coming over to study for french. He gave a thumbs up but didn't cede the room, so when she got there we set up on the couch in the living room.
Less than 3 minutes went by before he brought his laptop out to the living room and sat next to us, continuing to play. We looked at each other and decided the bedroom was clear, so in we went. This was much more successful as we got nearly 5 minutes before he came into the room, and sat on his bed to read a book as he'd done exactly zero times before. She suddenly needed to go visit a friend.
Never got an explanation, never got another chance with her, and never understood why I didn't think to lock the door.