The last time this happened to me was DEN-ORD when a 2-class 767 was swapped for a 3-class. I was in row 2 and thought I had scored a double. Unfortunately, the boarding-pass reader at the gate went "BONG" and I was downgraded to C tootsuite. But C was still nicer than 2-class F so I wasn't too disappointed.

The other time this happened was SFO-DEN on a 777 (2-class to 3). The "BONG" was ignored and we were comfortably seated in row 2. Two non-rev employees showed up at the 10-minute mark. Enter the gate agent, who took our boarding passes and returned them with "9A/B" hand-written in green marker. It got a bit hot then as I requested "real" boarding passes and asked why the error wasn't corrected at the gate as the experience was a bit embarrassing. The agent was NOT happy to be called out but I thought the whole thing was unprofessional to say the least.

Of course, row 9 was fine and the wonderful purser spoiled us with attention to make up for the lousy treatment on the ground.