My Citibike "key" often doesn't work. You know the drill: slide it into the slot, and nothing happens; or it blinks once, then the red light flashes; or some such malarkey. So then I go to the next bike... and the next. The reason, sometimes, is that the bike in the dock isn't properly docked; so if I just go through the motions of undocking the bike, I'll probably free the bike. Don't get me wrong, I have a valid key and always return whatever bike I take when I get to my destination. But I'm not the only one that's figured this out. This may explain some of the bikes just left around.