Overbooking: Be aware that airline flights may be overbooked. A person denied boarding on a flight may be entitled to a compensatory payment. The rules for denied boarding are available at all ticket counters.
Keep in mind that when they are looking to bump someone, if they cannot get anyone to volunteer to take a different flight, they will often look at those who have failed to properly check in or even who did not check in early or online before the flight.
Also, those who arrive late at the gate will often be the ones who are bumped.