An extraordinary warning from IBM CEO Arvind Krishna sent shares plunging – on pace for their worst day in the 115-year-old company’s history. IBM (IBM) shares plunged 24% in premarket trading. Its worst day ever was October 19, 1987 – aka Black Monday – when shares fell 23.7%, mirroring the worst day in the overall stock market’s history. In a preview of the company’s earnings (IBM is set to report its quarterly financial statement on July 22), Krishna said the company struggled over the past t...