It's happened more than once or twice on my flights. I don't think that these people deserve the benefit of a discreet request to move to coach, in fact I think they should simply be thrown off. They are trying to steal. No matter, in order to not ruffle those in F/C, I always try to be discreet (but very firm) in telling this person they must move once I've found out that they don't belong there. Amenities not withstanding, there is no possible way that ANYONE could mistake their coach seat for a F/C seat.