I've had an upgrade revoked twice.

Once I think I may have inadvertantly *caused* it to happen to other passengers. In that case, four members of our family were flying paid FC tickets. Due to IRROPS, we missed a connection in MSP and the gate agent rebooked us on the next flight, but in coach (without explanation or offer of FC seats on a later flight, he just handed us boarding passes and sent us on our way). Once I noticed that we were rebooked in coach, I did a bit of grumbling about being rebooked in Y when there were FC seats available for upgrade. The gate agent for the rebooked flight put us in those seats, but while we were boarding, there was a guy yapping on his cell phone about how he had notification that he had been upgraded, but it had been yanked somehow.