In whatever game, analogy, or "guideline," expect rule benders (or breakers- however you want to put it). You'll come to a time where you have this awesome game or activity planned all the way out to the specifics and see it going so well in your head, but then all of a sudden there will be this group who takes it to the next level. Whatever this level is, it allows them to get an unfair advantage and ruin it for the rest. Its fun and games until it comes to decision time.
With this said, be ready to be firm and decisive on whats "in" and whats "out." From my experience, its always better to error on the side of strictness than being lenient on game-demolishers.