Somebody once asked Niels Bohr why he had a horseshoe hanging above the front door of his house. "Surely you, a world famous physicist, can't really believe that hanging a horseshoe above your door brings you luck?". "Of course not," Bohr replied, "but I have been reliably informed that it will bring me luck whether I believe in it or not."
For the want of a nail, the shoe was lose; for the want of a shoe the horse was lose; and for the want of a horse the rider was lost, being overtaken and slain by the enemy, all for the want of care about a horseshoe nail.