From his desk in Lower Manhattan, a banker at Goldman Sachs thumbed through confidential documents — courtesy of a source inside the United States government. The banker came to Goldman through the so-called revolving door, the symbolic portal that connects financial regulators to Wall Street. He joined in July after spending seven years as a regulator at the Federal Reserve Bank of New York, the government’s front line in overseeing the financial industry. He received the confidential information, lawyers briefed on the matter suspect, from a former colleague who was still working at the New York Fed. As a reminder,...