I was given a first class seat on a US airline whist travelling on a staff ticket.
Shortly before take off the gate agent came on board, knelt besides me and discreetly told me that they had a last minute revenue passenger so I'd need to move. She went on to say that unfortunately there was no availability in J so I'd need to move back to Y.
The entire cabin watched as the flight attendant took back my glass of champagne and I retrieved my carry on from the overhead.
I made the walk of shame to the back of the plane convinced that everyone would have thought that I was a self upgrader.