There can be an open door to an empty shaft when the elevator is undergoing maintenance. This happens sometimes in the high-rise building where I work. Normally the area around the open shaft is blocked off with some kind of barrier during maintenance, but I suppose that the mechanics could forget to put up the barrier out of negligence or someone could disregard the barrier and walk past it. It's also possible that the people who fall into the shaft are the elevator mechanics themselves rather than passengers.