An excellent question, and I believe the most likely scenario (and most common one) is that they simply fail to prevent the plots they orchestrated out of sheer negligence every once in a while.
Some agent is grooming someone, leaves/gets promoted, the new handler doesn't quite understand what was going on and then all of a sudden they have a "situation" on their hands.
What evidence led you to think this is MOST likely - or in fact has ever happened in history?