TV Tropes is of course awesome and any nerd will happily waste time reading its various entries detailing the various weirdness that seems to occur and recur in disparate TV shows.
However, this is the funniest by far:
The Law of Conservation of Ninjutsu
In any martial arts fight, there is only a finite amount of ninjutsu available to each side in a given encounter. As a result, one Ninja is a deadly threat, but an army of them are cannon fodder.