The old man on the frontier lost his horse
Once upon a time, there lived an old man on the northern frontier of China. One day, his horse disappeared. His neighbors came to comfort him. But the old man was not upset at all. He said the loss might turn out to be a good thing. And he was right. A few months later, his horse came back with another horse that was even better. His neighbors came to congratulate him on his gain. But once again, the old man viewed the event differently. He said this "good luck" might turn out to be misfortune in the end. Strangely, he was right again. A few days later, his son fell from the new horse and broke his leg. But fortunately, since his son was lame after that accident, he was not chosen to be a soldier to fight in the following war so that he lived with family safely.
Nowadays people use "The old man on the frontier lost his horse" to comfort those who have some misfortune. It implies: Bad luck sometimes turns out to be good luck, and good luck can sometimes be bad.