In fact, I'm not sure anyone has ever asked that before; not in such a way, at least. PoorCynic is right - if the mission isn't finished according to the Rules, the agents get some sort of 'punishment'; reassignment or an awful mission. They always have to go back and fix things if it's possible.

As for outright failing a mission - being unable to kill the Sue, or something... I'm not sure. If something that dramatic happens I suppose there would be a larger force of agents sent in to do the job. That's rare, though. I wouldn't worry.

From what I have observed, whenever agents become too zealous and fail to gather enough charges before killing the Sue, then the Flowers usually assign them a punishment of sorts. The badfic used for this punishment is either tremendously terrible (we're talking Bleepfic at least) or it takes place in a continuum that would seriously infuriate the agent.

Or, sometimes, nothing happens at all. It's totally up to you, as the author.

*Goes to claim the Firefly fic.*