Jason Fried over at SVN:
I’d say “Why does it matter right now? I’m not racing anyone today. I’ll do it right at the meet this weekend.”
“I’ll tell you why it matters” he’d say, sternly. “You play like you practice. Practice sloppy and you’ll play sloppy.”
From http://37signals.com/svn/posts/3504-you-play-like-you-practice
I have seen this happen so many times, both in sports and in my professional life. Right now, my field hockey team is experiencing a very bad run : 9 losses and 2 draws out of 14 games played this year. Guess how much we practice? We don't.
The same is true with the projects we're working on. Once you've made a certain set of mistakes a number of times, you stop seeing them when you do them. They become a habit. Bad work habits are very hard to break.
An implicit message is the importance of having a coach who's there to point out your bad habits and force you to do things right. Whether while playing sports or in your job, having someone who is able to help you learn how to do things right is invaluable.