This does not sound like a good day at Citigroup. Chief executive officer Vikram Pandit resigned, effective immediately, early Tuesday, as did president and chief operating officer John Havens. Shares of Citigroup fell in pre-market trading on the news. The board of directors praised Pandit’s “leadership” but immediately appointed Michael Corbat as CEO. Sources told the Wall Street Journal that Pandit resigned following a clash with the company's board over strategy and operating performance. Pandit’s resignation comes just one day after the firm reported third-quarter earnings above what had been expected.