A huge fall in the Dow appears to have been caused after a trader inputting details hit the button for billion not million.
The Dow's overall rate fell by nearly a thousand points in an anomalous pattern that had the Nasdaq and New York Stock Exchange announcing they would cancel all trades more than 60 percent above or below market that occurred between 2:40 p.m. and 3:00 p.m. New York time.
According to CNBC the problem came when a deal involving Proctor and Gamble shares was incorrectly entered.
"We, along with the rest of the financial industry, are investigating to find the source of today's market volatility," Citigroup said in a statement.
"At this point we have no evidence that Citi was involved in any erroneous transaction."
Proctor and Gamble shares fell by over a third on the day's trading.
"We don't know what caused it," said Procter & Gamble spokeswoman Jennifer Chelune.
"We know that that was an electronic trade...and we're looking into it with Nasdaq and the other major electronic exchanges."
Update: US regulators discounted the possibility of trader error -- the so-called "fat finger theory" -- on May 8. CME Group's detailed report on the incident is available here.