I had a flight last month ATL-FLL where the FA came back after we were in the air and said I was the next person on the list for an open FC seat. I know the flight had checked in full for FC and I was on the flight from the Standby List. When I got on the flight at the SkyPriority Zone there was 0 FC seats available and my name was not listed as one of the 52 people on the upgraded list. The flight actually left the gate on time with an open FC seat.

You really can't complain when you are not expecting the upgrade.