I've been having a nagging feeling in the back of my mind about one of the players in the Fate game I'm running, but it only just clicked after today's session: his motives for RPing are in conflict with everyone else's motives - he's only there for the humour. Whilst humour is great, doing things like "I steal a chair from the royal palace" is out of character and senseless. I don't put all this work into creating a campaign only for one person to throw all that out the window and turn it into random shenanigans. It seems I'm going to have to talk to him.
Or maybe I'm just a ****ty DM.