WARNING: this will be true as long as the person does their checkin in person and the CO employee swipes their ID, before the computer has released all unclaimed seats to the gate.

had this problem back in July, final destination of BUD, meant no OLCI for me and when I got to the Kiosk and the CO person ran their ID at that exact same time 1hr 3 mins before departure time, the computer released the hold on all unclaimed seats, since I hadnt officially checked in my seat was released and all that was left was not a seat I was willing to sit in on a Full flight.

So if anyone checks in after the computer released the unclaimed seats the only place to get them will be the gate, most likely by the time you get there they will have alreday been reassigned