Quite frequently I end up with a paper ticket when I get upgraded. I have to book my tickets through a corporate travel agent, and for some reason the upgrade process screws something up. It's extremely annoying because it prevents me from checking in online, and nobody I've talked to (AS agents, corporate travel agent) seems to know what the problem is or how to fix it. It has caused me to miss flights on more than one occasion (due to arriving at the airport just under T-30).