Many hotels today require plastic keycards with magnetic strips to open your room door. They often give you a “Do not disturb” card that you can stick into the same card slot. No one on the hotel staff can possibly ignore this sign and open your door, because they would have to take the card out first, to unlock the door. It's a perfect “Do not disturb sign”, and if you use it, nothing can go wrong.
One day this week I came back to my hotel room, removed the “Do not disturb” card, unlocked the door, put the card back in the door and entered my room. I locked the extra inside lock and felt perfectly safe.
Until I noticed that I was still holding the “Do not disturb” card in my hand. Where was my plastic room keycard? It was outside the room, stuck in the door slot, just waiting for someone else to use it.