塞翁失马，焉知非福 — “A seemingly bad event may turn out to be a good thing”

In ancient times, an old man living near the frontier lost his horse, which ran away into the territory of the nomadic people. His neighbors came to console him, but the old man said, "How do you know this isn't a good thing?"

A few days later, the lost horse returned, bringing with it several fine wild horses. The neighbors congratulated him, but the old man said, "How do you know this isn't a bad thing?" Indeed, his son enjoyed riding the new, spirited horses and one day fell off, breaking his leg.

The neighbors again expressed their sympathy, but the old man calmly said, "How do you know this isn't a good thing?" Some time later, a war broke out, and all able-bodied young men were conscripted into the army. Many were killed in battle.

However, the old man's son was spared from military service because of his leg injury, thus avoiding the war and its dangers.