捡漏

捡漏	jiǎn lòu	to repair a leaky roof / (dialect) to find fault; to nitpick / (slang) to score a bargain (esp. when the seller is unaware of the item's true value) / (slang) to take advantage of an unexpected opportunity
捡	jiǎn	to pick up; to collect; to gather
漏	lòu	to leak / to divulge / to leave out by mistake / waterclock or hourglass (old)