Funny - after reading this thread a few days ago, I was headed from PDX to ATL in J.
I boarded, sat down, got my drink and right before door closed, nice lady and her husband come up and tell me I'm in the wrong seat. My reflexes almost kicked in to put up a fight, thinking they're trying to boot me from my seat.
I look at my boarding pass before standing my ground, and realize I'm in 2c instead of the 3c I was assigned.
Guess I was tired. Sigh.