Saw it done once when on a Lufthansa Frankfurt - Washington Dullas A340 flight in Business class with my two boys . Very light Business load , stewardess just checked our boarding cards as the tally didn't match and hoiked out some guy sitting next to someone else chatting away , obviously hoping to get away with it . He was asked to go back to Economy . Very nicely done , I must say . I know my lads would never do that now !!! Don't people understand headcounts ?