Practice does not make perfect.

I took karate lessons when I was in middle school. Frequently the sensei would say "Practice does not make perfect." It was always when he saw one of the students performing a move sloppily or wrong. As he was kindly correcting the student, he would follow-up with "Perfect practice makes perfect."

I always thought that was sort or profound, and made it a lot of sense. I was only tweleve then, but it still rings true today.

One practices to improve and become better at a skill. Obviously when you start practicing something new -- karate, a musical instrument, mathematics, whatever -- you won't be perfect right from the beginning. You still need to try your hardest to do it correctly when you practice, otherwise your practice won't make you perfect. It will make you be flawed in whatever way you practice imperfectly.

Is there anything you can practice incorrectly, and still become perfect at it? If you can change the "rules of the game" as you go along, then perhaps. But, I think it's unlikely that someone who practices playing the trumpet incorrectly will invent a new kind of jazz.