Had a great experience that way flying OAX-MEX on New Year's Eve, 2005. I was on a paid economy ticket, needing the miles to complete EXP, upgraded from MEX onward to BOS on AA but couldn't upgrade the leg on Mexicana iron. My SO was on a business/first award covering all three legs.

There was plenty of space in the front cabin on the first leg, including the seat next to her, so I asked the FA if I could sit there. He turned his back to the entrance so nobody could see in front of him, said in a loud voice so everyone back there would hear him "I'm sorry, sir, you must sit in your assigned seat in economy," while gesturing madly in front of his body (which nobody else could see) for me to get into that frackin' seat before people wondered why he was blocking the aisle. Needless to say, I did.