The BSE benchmark Sensex pared some of the gains and slipped 45 points after topping the 20,000mark in the morning trade. It dropped to 19,861 points in the late morning deals. The wide-based National Stock Exchange also failed to sustain the 6,000 points mark and traded 18.60 points down at 5,962. Analysts said emergence of profit-booking by funds as well as retail investors mainly reversed the early gains.