塞翁失馬，焉知非福

From a tale in Huai Nan Zi:

夫禍福之轉而相生，其變難見也。
近塞上之人有善術者，馬無故亡而入胡。人皆吊之。
其父曰：「此何遽不為福乎？」
居數月，其馬將胡駿馬而歸。人皆賀之。
其父曰：「此何遽不能為禍乎？」
家富良馬，其子好騎，墮而折其髀。人皆吊之。
其父曰：「此何遽不為福乎？」
居一年，胡人大入塞，丁壯者引弦而戰，近塞之人，死者十九，此獨以跛之故，父子相保。
故福之為禍，禍之為福，化不可極，深不可測也。 [Classical Chinese, trad.]

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夫祸福之转而相生，其变难见也。
近塞上之人有善术者，马无故亡而入胡。人皆吊之。
其父曰：「此何遽不为福乎？」
居数月，其马将胡骏马而归。人皆贺之。
其父曰：「此何遽不能为祸乎？」
家富良马，其子好骑，堕而折其髀。人皆吊之。
其父曰：「此何遽不为福乎？」
居一年，胡人大入塞，丁壮者引弦而战，近塞之人，死者十九，此独以跛之故，父子相保。
故福之为祸，祸之为福，化不可极，深不可测也。 [Classical Chinese, simp.]

Fú huòfú zhī zhuǎn ér xiāngshēng, qí biàn nán jiàn yě.
Jìn sài shàng zhī rén yǒu shàn shù zhě, mǎ wúgù wáng ér rù hú. Rén jiē diào zhī.
Qí fù yuē: “Cǐ hé jù bù wéi fú hū?”
Jū shù yuè, qí mǎ jiāng hú jùnmǎ ér guī. Rén jiē hè zhī.
Qí fù yuē: “Cǐ hé jù bùnéng wéi huò hū?”
Jiā fù liáng mǎ, qí zǐ hǎo qí, duò ér zhé qí bì. Rén jiē diào zhī.
Qí fù yuē: “Cǐ hé jù bù wéi fú hū?”
Jū yī nián, húrén dà rù sài, dīngzhuàng zhě yǐn xián ér zhàn, jìn sài zhī rén, sǐ zhě shí jiǔ, cǐ dú yǐ bǒ zhī gù, fù zǐ xiāng bǎo.
Gù fú zhī wéi huò, huò zhī wéi fú, huà bùkě jí, shēn bùkě cè yě. [Pinyin]

It can be difficult to foresee the twists and turns which compel misfortune to beget fortune, and vice versa.
There once was a (father), skilled in divination, who lived close to the frontier (with his son). One of his horses accidentally strayed into the lands of the Xiongnu, so everyone consoled him.
(But) the father said, “Why should I hastily (conclude) that this is not fortunate?”
After several months, the horse came back from the land of the Xiongnu, accompanied by another fine horse, so everyone congratulated him.
(But) the father said, “Why should I hastily (conclude) that this can not be unfortunate?”
His family had a wealth of fine horses, and his son loved riding them. One day (the son) fell off the horse, and broke his leg, so everyone consoled (the father).
(But) the father said, “Why should I hastily (conclude) that this is not fortunate?”
One year later, the Xiongnu invaded the frontier, and all able-bodied men took up arms and went to war. Of the men from the frontier (who volunteered), nine out of ten men perished (from the fighting). It was only because of (the son's) broken leg, that the father and son were spared (this tragedy).
Therefore, misfortune begets fortune, and fortune begets misfortune. This goes on without end, and its depths can not be measured. (Wiktionary translation)

塞翁失馬，焉知非福

1. Every cloud has a silver lining. / A setback may turn out to be a blessing in disguise.