s122604 points out a CNBC story according to which
"the catalyst for today's extraordinary price swing (at one point the Dow lost almost 9 percent in less than an hour) may have been because a trader entered a 'B' for billions instead of an 'M' for millions on a trade of Procter and Gamble: 'According to multiple sources, a trader entered a "b" for billion instead of an "m" for million in a trade possibly involving Procter & Gamble, a component in the Dow. (CNBC's Jim Cramer noted suspicious price movement in P&G stock on air during the height of the market selloff).' Unbelievable there are no safeguards to protect against this."