You know how they have a rule that you can't board before your zone is called (say F is zone 0 or zone 1, depends on the airline). However, you can always board after your zone is called. Someone in F can be the last to board the plane, and the gate agent doesn't care. They do care if someone in zone 9 tries to board when it's time to board First Class. Anyway, here's what they should do when the standby list is longer than the number of seats left on the plane: if you miss your boarding zone, you miss your flight. The computer will get mad if you scan your boarding pass and the gate agent will have to tell you that you've missed your flight because they already gave away your seat to a standby passenger. Please see customer service for re-booking, and next time don't be late for your boarding zone. In other words, instead of clearing the standbys after everyone else has boarded the plane, clear them one zone at a time. That includes First Class. That means if you have an F ticket and you fart around and don't get on the plane when it's time for First Class to board on a full+ flight, the second F is done boarding, they will op-up someone and give them your F seat.