亡羊补牢 (wáng yáng bǔ láo): Mend the fold after a sheep is lost / take remedial action — Meaning & Stroke Order

Although there were oversights in the early stages of the project, it's much better to mend the fold after the sheep is lost than to do nothing.

You're only realizing the health issues now, don't you think this is a case of mending the fold after the sheep is lost?

He was not discouraged by the failure; instead, he adjusted his strategy with the determination to mend the fold after the sheep was lost.

This is not about mending the fold after the sheep is lost; it's a fundamental mistake that should have been avoided from the start.