(CNBC) Warren Buffett doesn’t have many bad days, but he really ought to consider going on vacation whenever IBM (IBM)reports earnings. The company’s shares fell $8.96 at Tuesday’s open after a mixed earnings report , costing the Oracle of Omaha some $712.9 million. His conglomerate Berkshire Hathaway owned 79.57 million shares of IBM as of March 31, according to the most recently available filings, making it one of his largest investments…