I witnessed someone getting caught doing this the other day.

I was in F on a flight this friday from SJC to LAX. A passenger who was just boarding noticed a friend/coworker/acquaintance sitting in F with a empty seat next to him. The person continued on his merry way to Coach.

After the flight took off, the coach passenger came up to F and sat down next to his friend. After a few minutes, a FA came up to the person and politely informed him that they were about to begin the coach beverage service and that he needed to return to his seat. He made some feeble argument that he didn't care for any beverages since it was such a short flight and that he "just needed" to talk to his friend. The FA again politely informed him that he needed to go back to his seach in coach. If they wanted to talk, his F friend could go back to coach and sit with him.

The "F thief" heed and hawed about having tried to upgrade at the gate etc. and that he would have paid for an F seat if their was space (F was only half full). The FA informed him that if he wanted, the FA could process the difference in price between his ticket and the F price. They made such a fuss and decided to pay the F, "just to show the FA that they weren't trying to steal a $%^&*()# F seat!) The difference in price was processed by credit card on the spot. The coach guy then went back to his seat in coach and brought his bags up to F, making all sort of noise and a stink the whole time, slamming the overheads after moving his bags.

Oh well, he was probably a privlidged/spoiled kid when he was young who got whatever he wanted and couldn't believe that he had to pay for an F seat.