If small investors needed any more reason to be disgusted with the stock market, they got it Wednesday.

At 9:30 a.m., the broad market averages like the Dow Jones Industrial Average were placid as investors held their breath waiting for the afternoon announcement on interest-rate policy from the Federal Reserve. But beneath the macro-calm was micro-turmoil, as nearly 150 stocks traded on up to 20 times their normal volume—and many fell 10% or more in price in a matter of seconds after the market opened for trading, before...