I had a passenger a while back who started making a fuss because there were a few empty seats in business class and simply couldn't understand why he couldn't sit in one.
I popped out very politely asked him if he had ever bought something only to find out that one of his mates had bought the same thing much cheaper elsewhere? He replied "yes". I asked him how that made him feel? He said "very annoyed". I then asked him how he thought the rest of the pax who had paid the full business fare would feel knowing that he had just paid economy? He went off to his seat like a lamb!
One other story; a certain very well known journalist stormed onto the plane and demanded an upgrade to business. The cabin crew politely explained that there were rules etc etc, he started on the "do you know who I am?", "I demand to see the captain", "I'll have your jobs for this". . . In the end I was called out, had a polite chat, checked his boarding card and said that of course he could sit in Business. I thought that the Cabin Crew were going to lynch me there and then, until I pointed out when he was seated that check-in had actually already upgraded the guy to 1st!! He was so intent on getting an upgrade from economy to business he hadn't even looked at his boarding card!!!
Edited - Spolling mistok!