A sharp spike in oil futures before lunch yesterday prompted volatile trading in Hang Seng Index futures and a sell-off in blue chips. What started as a quiet session, with a downward drift in prices, abruptly changed with sharp selling in the June contract, forcing a breach below 12,000 points and a wave of stop-loss selling. 'It was very sharp. We initially wondered if it was the result of an error,' said Matthew Long, an associate director of equity derivatives sales at Macquarie Equities. The contract ended the session 376 points lower at 11,859.