I've never heard a quote so abruptly true as this:
"Never interrupt your enemy whilst he is making a mistake" - Napoleon Bonaparte (Emperor of France, crica 1800).
Others:
"When life is on the line, and it is a matter of death, never go against a Sicilian" - Unknown.
"Blood saves lives, sweat saves blood, and brains saves both" - Erwin Rommel (German General/Field Marschall of the 'Afrika Korps', 1941-43).
"Nothing on earth travels swifter than rumour" - Latin Proverb.