Don't put up with that! Tell a crew member and remove it. Be sure that the person is in Y - nothing more embarassing than that particular mistake.

If the Y passengers thought that they were being cute or clever then they might end up having the item checked if there is no room left.

I'll share something with you - 9 out of 10 times that this happens who are "elites" in card if not in manners. The sort of person who pulls that stunt thinks that they are entitled to be in CW/CE whatever fare they have paid.

I think that it is a far more acute problem in Club Europe as space is ridiculously short. You tell everyone that they can bring two pieces and then give them nowhere to put it. Sheer Genius!