Another Zen story, this one retold in Buddhism: Plain & Simple by Steve Hagen, p42:

The situation we always live in is like the wise Chinese farmer whose horse ran off. When his neighbor came to console him the farmer said, “Who knows what’s good or bad?”

When his horse returned the next day with a herd of horses following her, the foolish neighbor came to congratulate him on his good fortune.

“Who knows what’s good or bad?” said the farmer.

Then, when the farmer’s son broke his leg trying to ride one of the new horses, the foolish neighbor came to console him again.

“Who knows what’s good or bad?” said the farmer.

When the army passed through, conscripting men for war, they passed over the farmer’s son because of his broken leg. When the foolish man came to congratulate the farmer that his son would be spared, again the farmer said, “Who knows what’s good or bad?”