bdshobel .. I have none personally but it was posted in a thread here only a week or so back. Not sure which board sadly, but it was someone here SAW it happen, it seems.
"Please either pay for the difference between your paid coach or First seat or go back to your coach seat, and the person asked did apparenlty pay the sum asked despite loud mutterings and protest!
Someone else may recall the link. Maybe even have been Premex who posted it .. not certain.
"I still maintain that no airline has ever tried to charge a passenger for sitting in the wrong seat. Does anyone have evidence to the contrary?"