hmmm...I think that if you physically seat yourself in a higher class cabin than your boarding pass indicates, and you should be required to pay the difference on the spot, but only if you truely knew that you were not supposed to be in a higher class cabin. I do know that some people don't fly often and do not realize the cabin classes. If a person honestly makes a mistake, and the FA realizes this, the FA should just ask the person to return to their assigned seat.

When I do get to fly first(rarely these days)I always leave my boarding pass on my arm rest until well after departure so the FA will have no doubt I am in my assigned seat.