Goldman Sachs got a disconcerting update a year after buying its second coal mine in Colombia: “Certain operational issues have arisen,” commodities executives reported.

That was putting it mildly. Local women and children had formed a human blockade to protest labor issues, shutting down production. Coal prices had dropped 20% in three years, and another 6% decline could permanently impair the value of Goldman's investment, the executives told directors in late 2013.