Had one case of an upgrade which is a little unusual. Flying from SNA to DEN on UA, my travel agent had unknowlingly to me requested a complimentary Premier 1000 mile upgrade. I got to the gate and the agent told me that my upgrade had gone thru. I mentioned that I did not want to upgrade and that I did not have a certificate. However, UA had in the meanwhile filled my coach seat. As the agent looked thru the computer, she decided that since coach was now oversold, she would have to put me in First with no certificate. I told her I would be glad to send the certificate, she said that it would not be necessary. It occurred to me that you could use this technique over and over, but each of the next few times I was waitlisted for an upgrade, the agent asked me if I had my certificate, which leads me to believe that the agent at SNA had entered something in the computer. As a result, I have not nor will not ask for an upgrade without a certificate.