I'm not a lawyer; I just sound like one sometimes, from years of working closely with lawyers, I suppose!

Anyway, proving that someone intentionally sat in the wrong class might be impossible if the person was never confronted. (And if the person were confronted, he or she undoubtedly would have moved.) The person would just claim that it was an innocent mistake.

In any event, the airline has an obligation to mitigate its damages. If it sees someone "stealing" services, it has to take reasonable steps to stop the loss -- such as confronting the individual. It cannot just let the loss occur, on the assumption that they can just send the person a bill afterward. Imagine if Wal-Mart let shoplifters steal whatever they wanted but just billed their credit cards (this is totally fantasy, of course). If you don't protect your property, a thief could claim that it appeared to be abandoned, and abandoned property is free for the taking.

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?

Bruce