Alaska Airlines.
Upgraded to first class on SEA-PDX-ONT.
PDX was a layover, no need to exit airplane.
In PDX, AFTER the plane boarded, I was told I had to get off the plane, because a passenger that had purchased a first class ticket needed my seat and there were no more seats available in coach.
I tried to politely protest, and was immediately informed that security could come help me with my bags.
I had to get the next flight.

Never flew Alaska Airlines again.