Domestic benchmark equity indices opened on a weak note on May 16, following a sharp rally in the previous session. The Sensex started 138.11 points or 0.17 per cent lower at 82,392.63, while the Nifty 50 began with marginal gains of 2.55 points or 0.01 per cent at 25,064.65. However, markets slipped soon after the opening bell, with the Sensex falling as much as 250 points and the Nifty declining over 50 points.