I had a situation happen to me last year where I was given the certificate. I had been confirmed for the upgrade at the time of purchase and somehow when I checked in my reservation had been cancelled. I was flying ORD-IAH-SAN Apparently someone with a similar name checked in at LGA and the agent in LGA cancelled out my PNR. I don't know how this could have happened since one could assume they would check credit card and OnePass numbers to see that they didnt line up. But somehow in the process of the agent attempting to rebook me on the flight (as now it was oversold), another OnePass member was upgrade (I was at a higher status at that time). I was upset not so much that I was downgraded, but that my ticket was cancelled because of someone not paying attention to what was going on. The agent in ORD was apologetic but I still was bothered that the return portion of my trip was cancelled and the flights were all sold out at that point.