百勝難慮敵，三折乃良醫

(a line from a poem by the Tang poet Liu Yuxi 劉禹錫|刘禹锡[Liu2 Yu3 xi1]) one gains very little insight into one's enemy from a hundred victories, but he who breaks his arm three times will be a good doctor; (fig.) one learns more from one's failures than from one's successes