Over-booking of flights used to be a huge problem back in the 90's and earlier. You'd get on the plane, someone would be in your seat already, and that's when you found out you didn't get to fly on that flight.
Airlines figured out this pissed people off and started counting tickets before letting people onto flights. If they had too many people waiting to board a flight, they would make an announcement and start offering free stuff to people willing to be bumped or take a later flight (this still happens in rare circumstances today but used to happen on most flights for a few years).
I remember listening to a gate agent threatening to call the cops on a college age guy who was picking up his third free ticket of the week for accepting being bumped off an Oakland to Las Vegas flight. He'd apparently figured out that that flight was always overbooked (which wouldn't have taken a rocket scientist given the high rate of overbooking at the time), and the only reason he got caught was the same agent kept getting assigned to work the counter for that flight often enough to remember his face.
Interestingly, I also flew on a nearly empty flight once during that era. The pilot came on the loud speakers and said "the next time you are wondering why flights are over booked, remember this flight - this plane was booked to 120% capacity (or some other number well over 100%) for this trip."