Sensei Nishiyama repeated that sentence many times, at least once a week. It was enlightening for me, I always try to be perfect and now he tells me to make mistakes, at first it seem contradictory to the concept of “always do your best, as if last chance of your life”.
But in reality you can never do your best if you do not accept mistake, you will always be inhibited, will confirm, will do things carefully and will not be able to give your whole being into the technique.
Sensei used to say: “Once you go, only god knows win or lose”, "don’t worry about results".
Only when we accept mistake we can go into unknown areas, away from the habitual and convenient.
Welcoming mistakes and correcting them, this humility is the only path for true learning.