To add to the original post: I was flying NW, LAX-LAS. No bags(since I wasn't planning on going anywhere).
I boarded the flight early. They took my ticket. About half way down the jetway I just turned around and walked back to the agent. I think I told her I had left something at the newstand (Or maybe I needed to buy a coffee). Can't remember. She said, you have plenty of time until departure. And then I just walked out of the terminal, back to my car in short term parking. I got credit for the segment, reached the level, viola! EQMs! This was before 9/11 and my guess is around 1999.
I used to always get on flights, stow my bag, then get off and wait for the flight inside the terminal. Last call, I'd board. (I used to get claustrophobic). But that is no longer an option these days. bummer.
On a side note, back in 1990, when I flew Continental(this seemed to only work with them), i would always send in my boarding pass even when I was credited for the flight segment. For about a two year period, they would apply miles to my account without checking to see if my account was credited. Sometimes I could xerox the boarding pass and get credit for the same segment over and over...
That ended when someone woke up. But they never took the miles back!
I still have some continental miles from that period (early 90s). Ethical? Nope. Do I feel bad about getting something I didn't earn? A little. Would I do it again if it worked? Probably not. I was younger and more cavalier then...