The Sensex at the Bombay Stock Exchange fell 20 points, to 15,848, in opening trade of the special trading session today as investors resorted to mild profit-booking.Later, Sensex remained subdued, and stood 27 points in negative territory, at 15,840 a short while ago. The stock market is 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.