I was always told it never happened, but I guess I was just lucky!

I was traveling ATH-LHR on BA in coach. Checked in early and got a coach boarding pass. Nothing said at Terraces and then 2 minutes before boarding, "Will London passenger X please come to the the gate." And there is a bus class boarding pass. Wow.

The seats are smaller than AA domestic first (5ish across), the food about the same, and the service a bit better.

Some days the upgrade god smiles!