Hoist with his own petard.

One of those phrases which most people are familiar with, but are unsure as to what exactly it means. It comes from Shakespeare (who else?) and means to be blown up by your own bomb, or metaphorically, to be caught out by a situation you had created yourself - the sort of thing which always makes you smile and think "serves them right".  Here's a great one - the deputy leader of South Lanarkshire Council was instrumental in closing all the public toilets in the council's area earlier this year as a cost-saving measure.  You can see where this is going, can't you?  He found himself "caught short" while waiting for a taxi, and went up an alley to relieve himself, was seen by the police and given an on-the-spot fine.  Nice one!