Happened to me twice.

The first time, I was a non-rev just settling in my first class seat when I heard my name called. Sure enough, a revenue pax needed my seat and off I went.

The second time, I was discussing a bump at the gate, and the CSR wasn't sure. I had a first seat, but by the time I made up my mind, it had been given away and I wound up in a middle seat in coach. As we sat at the gate, the CSR came on board - she looked at me, I looked at her, raised my eyebrows quizzically, she gave a slight nod, and I bounded out of my seat like the very hounds of hell were after me.

Worked out great - for some reason she couldn't print the VDB comp ticket for the right amount, so she printed two of the ones she could print and I think I wound up with about 40,000 miles out of the deal, along with a meal voucher.

Ahh, good times.