Once Upon A Time the no-show high-paying flexible ticket holders used to be dealt with by something called "standby". When the flight was notionally full, you got sold a "standby" ticket, which entitled you to stand by the bottom of the steps, wait until no more confirmed passengers turned up, then get into the aircraft.
I was never denied boarding on a standby ticket, but if I had been
(a) it would have happened before I'd got onto the aircraft
(b) I wouldn't have been upset, because I knew exactly what I was buying.