Stocks experienced their worst day since June, with a sudden plunge Thursday in big tech shares such as Apple and Amazon driving the S&P 500 index down 3.5% and the Dow Jones Industrial Average down 2.8%. The dizzying drop in prices follows a string of record-high trading days, largely fueled by a few superstar tech stocks, such as Facebook, Apple and Amazon. It was those same high-flying tech companies that led the drop in the market, with Apple down by 8% and Microsoft losing more than 6%. The electric carmaker Tesla, which has become a favorite of day traders, lost 9%.