When David was preparing for battle, he picked up 5 smooth stones. Why five? If he had faith, why not 1? For that matter, why bring any weapon?
How many people train in self-defense expecting that groin kick to end the fight? What if you miss? What if he doesn't care? Better have a back up plan.
In your own training, you must prepare for Murphy to show up - as in Murphy's Law. If anything can go wrong, you must be prepared for it to go wrong at the worst possible time. Get your backup plan ready.