A quote from the Art of War that might have some baring on Agile development techniques:
31. Water shapes its course according to the nature
of the ground over which it flows; the soldier works
out his victory in relation to the foe whom he is facing.
32. Therefore, just as water retains no constant shape,
so in warfare there are no constant conditions.
But then again, we all know war is agile, right?
Or another quote I heard (source unknown, original language Swedish):
Meeting all requirements in programming is like walking on water. It’s easy if the water and the requirements are frozen…