American Airlines guarantees its top-level elites (Executive Platinum) a full-fare main cabin seat on any flight if they book at least 24 hours in advance, even it has reached (or passed) its overbooking threshold. Overbooking is a statistical game, after all. Booking one more person just increases the odds that they'll have to VDB someone by a small amount. (If a plane holds 250 people and they sell 260 tickets, the chances of 251+ passengers showing up and wanting to fly don't increase much if they sell 261.) AA may not be the only airline to do this, officially or as an unpublished benefit of top-tier status.

You didn't say what airline this happened on, but I wouldn't be surprised to hear that any airline's best corporate customers get similar consideration.