Two ways...

1). Pax is waiting in boarding area for his/her upgrade to come through. Once the upgrade clears, the Pax boards the plane only to find the 1st. class bins full.

2). Some 1st. class Pax board the plane 10-15 minutes before departure. Coach has already begun boarding and often times have dumped thier bags in the 1st. class bins. When the 1st. class PAX boards the plane, there is no luggage space in first class.

There are very FEW benefits left for 1st. class passengers any more, other than a comfortable seat and early boarding. Having a place for my luggage is one of the perks that I relish the most. I'm not trying to sound "territorial", but if I paid for a 1st. class ticket, or earned it on upgrades for flying tens of thousands of miles per year, I want preferential treatment when it comes to overhead bin space.