亡羊补牢 — “Lose sheep mend pen”

This story originates from 'Strategies of the Warring States'. During the Warring States period, a minister named Zhuang Xin from the State of Chu warned King Xiang that his excessive indulgence in luxury and neglect of state affairs would lead to the downfall of the capital, Ying. The King, angered, accused Zhuang Xin of spreading malicious rumors. Zhuang Xin calmly insisted on the gravity of the situation and, with the King's permission, left to seek refuge in the State of Zhao, wanting to observe the unfolding events.

Just five months after Zhuang Xin's departure, the State of Qin indeed invaded Chu, forcing King Xiang to flee to Yangcheng. Realizing Zhuang Xin's words had been prophetic, the King immediately sent for him to ask for advice.

Zhuang Xin responded sincerely, 'I have heard that it is not too late to think of a hunting dog when you see a rabbit (meaning to prepare for a chase when traces of prey are found), and it is not too late to mend the sheepfold after a sheep has run away.' The idiom '亡羊补牢' (mending the fold after a sheep is lost) is derived from these lines, conveying the meaning that it is never too late to take corrective action after a mistake or loss has occurred to prevent further damage or to rectify the situation.