"At the front desk, I am a god of instant karma, and one of my other weapons is the "key bomb": When I check you in, I program a single "initial key," then start over and cut a second "initial key." Either one of them will work when you get to the room. Slide one in; you get the green light, and as long as you keep using the very first key you slipped in, all will be well. But chances are you'll pop in the second key at some point, and then the first key you used will be considered, as far as the dumb-... lock is concerned, invalid. At some point after that, you will be locked outside your room, jamming your first key into the slot, fighting that damn red light."
More at this link:
http://lifehacker.com/5972701/confes...-hotel-insider