Well, let me tell you what happened to me . . .
Two stories.
One: About 10 years ago I was visiting a friend in San Francisco on a US award ticket (coach, low mileage award). Her husband worked for an air freight company, and somebody at USAir owed him a favor. He called his friend that morning as I left for the aiport. When I got to the gate, I told the GA that I thought I was maybe supposed to be upgraded. In the usual harsh, sneering USAir way, she started to say, "No, that would be an extra cost . . ." then she saw something on the res, and I was in first. She didn't utter a word as she issued the boarding pass.
Two: Only last spring, on a cheap coach ticket to Madrid, I was sitting in the gate waiting for my PHL-MAD leg jabbering on the phone to a friend. I was right next to the podium. As I'm chatting the GA walks over and says, "Are you travelling alone sir?" I looked up and smiled and nodded. "Can I have your boarding pass, please?" I handed it over. I was having such a good time with my conversation that I hardly thought about it. Moments later she returned with a new boarding pass. It was like 2C or something. I gasped; told my friend with a chuckle, "I think I just got upgraded to first!" After I got off the phone, I checked with her, and thanked her. I think I was chosen because 1) the flight was full and there was one empty seat in first, and I was alone 2) I was conveniently close to the podium 3) I was dressed decently, in khakis and a dress shirt. This will probably never happen again, but it sure was nice on that long transatlantic flight . . . .