Years ago, I received a post-boarding upgrade on United -- and the agent forgot to take my certs. Oops!

Within the last year, sympathetic AA agents have come to my seat on two occasions and moved me to the front. One time they had told me to watch for it (and the stickers were taken from my upgrade account) -- the other was a complete surprise.

Now that I think of it, on the day of the surprise upgrade, I had been waylayed with a delayed flight that was to get me home well after midnight -- probably didn't hurt that I asked for cab fare (which I was told they couldn't provide).