Rookie mistake

We all make mistakes, me included. Years ago at a new job I screwed something up and told the boss...he said good, because if I had not screwed up within the first few weeks he would have fired me because He figured me the newbee was hiding mistakes.
A friend of a friend...a very experienced rock climber. clipped his harness onto the rope for a 800 foot rappel down a cliff face...well he wasn't clipped onto the rope. He made a rookie mistake. He jumps off to start the descent, and off he goes. That was the end of him.