"It takes 20 years to build a reputation and five minutes to ruin it. If you think about that, you'll do things differently." - Warren Buffet
Careless mistakes can prove very costly. We still promote taking chances and making errors, because they provide an important learning experience, but you can only embrace mistakes so much. There's a balance that needs to exist, where you take calculated and intelligent risks with the chance that you'll fail. You just don't want to fail so terribly that you'll tarnish your reputation and never get another chance. Mistakes are great when you can learn from them, but if you don't get another chance to do so they're not worth the risk.