Stocks had their worst day of trading since the Sept. 11, 2001, terrorist attacks Tuesday, hurtling the Dow Jones industrials down more than 400 points on a worldwide tide of concern that the U.S. and Chinese economies are stumbling and that share prices have become overinflated.

The steepness of the market's drop, as well as its global breadth, signaled a possible correction after a long period of stable and steadily rising stock markets that had not been shaken by such a volatile day of trading in several years.

A 9 percent slide in Chinese stocks, which came a day after investors sent Shanghai's benchmark index to a record high close, set the tone for U.S. trading. The Dow began the day falling sharply, and the decline accelerated throughout the course of the session before stocks took a huge plunge in late afternoon as computer-driven sell programs kicked in, and also as a computer glitch caused a delay in the recording of a large number of trades. More from Forbes