To mend the pen after sheep dies
When, after experiencing some sort of issue, someone takes timely steps to ensure that they don't face that issue again
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这次试验没有成功，我们应该亡羊补牢，认真总结教训 | Zhè cì shìyàn méiyǒu chénggōng, wǒmen yīnggāi wángyángbǔláo, rènzhēn zǒngjié jiàoxùn
The experiment failed this time around. We should learn from this failure and take steps to ensure it doesn't happen again
[Source]
墙虽然坏了，但只要及时补，就能做到亡羊补牢 | Qiáng suīrán huàile, dàn zhǐyào jíshí bǔ, jiù néng zuò dào wángyángbǔláo
The wall is broken, but if it's quickly repaired then this can be prevented from happening again
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《战国策》(Zhàn Gúo Cè, Strategies of the Warring States), a work which chronicled the Warring States Period (475 - 220 BC)
In ancient times, there lived a man who raised sheep. One morning, he went to let his flock out to graze, and he was astounded to discover that one of his sheep had gone missing. After investigation, he discovered that the sheep pen had a very large hole in it. He suspected that a wolf had been able to crawl inside the pen at night and drag his sheep away.
When his neighbors found out, they all urged him to fix the hole immediately. However, he didn't see a point in doing that, and he asked rhetorically, "The sheep is already gone. What is repairing the hole going to do?"
The next morning, he woke and found that yet another sheep had gone missing in the same fashion as the first one. He immediately regretted that he hadn't listened to his neighbors, and he quickly moved to repair the hole. From then on, the wolf was unable to take any more sheep.