The Sensex at the Bombay Stock Exchange fell 19 points, or 0.1 percent, to 15,849, in the special trading session today as investors resorted to mild profit-booking.The Nifty at the National Stock Exchange fell 7 points, or 0.2 percent, to 4,747. The stock market was open for 90 minutes for special trading from 11:15 a.m. to 12:45 p.m., today, to update the stock exchanges' software infrastructure.