百胜难虑敌，三折乃良医

bǎi shèng nán lǜ dí , sān zhé nǎi liáng yībaishengnanludi,sanzhenailiangyi

1. (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
2. (fig.) one learns more from one's failures than from one's successes

Study it in Yanamino