In the good old pre internet days my travel agent would frequently put me on the "wait list" for some flights I needed to take but space was not available at the time of booking. I often joined several other hopefuls at say No 4 on the list, and I can't remember not eventually being confirmed for any flight. Of course that was in the days when travel agents could delay ticketing until the client was about to fly, and could hold multiple unpaid bookings for several flights on the same route for the same person. I may be completely wrong but I suspect that this is no longer possible?