There's something satisfying about watching someone who decides to "upgrade" themselves to first class get caught.
Here's the thread on Flyertalk:
I saw someone try this on an Alaska flight once. She boarded early and sat down in a first class seat. Then the passenger who was assigned that seat got on, so she shuffled off to another fist class seat. Then the passenger assigned to the new seat she grabbed got on and she grabbled a third seat.
By this time the flight attendant noticed, checker her boarding pass and excorted her to her coach seat in the back.