Okay, yes, I see what you're getting at now. Most of the examples I gave regarding not remembering or knowing a piece (jazz players) had all put in thousands of hours of practice in order to be able to improvise that well anyway.

Still, you only learn to perform by ... performing. That's where, for a new performer, occasionally getting up for a floorspot and making an a**e of it helps. You never want to mess up so badly again, and it spurs you on to polish your act.