Probably not quite a DYKWIA but travelling from MAN-LHR a couple of days ago, the gentleman travelling in 1A was the last to board. He opened up the overhead and took out the bag and hat box that were in there in order to put his own case in.
He then proceeded to unsuccessfully try and ram the other items in which it became clear belonged to a member of the cabin crew. Said cabin crew member then came and took the items from the gentleman and began looking for somewhere else to store them.
What amazed me was that this was not a case of somebody making a fuss. There were no words exchanged at all. The guy just assumed he had the right to remove somebody else's possessions and place his luggage in their place because he was in 1A and wanted his bag right above him. I know some people get frustrated at crew using the overhead but it just struck me as incredibly rude to remove somebody else's bags to make space for your own without even asking them.
On my return yesterday evening I was also amazed by the number of people who seem to get personally offended when one item of their carry on is given the yellow tag.