Oil prices dropped more than $3 in less than a minute late in the trading day on Monday, just as trading volume spiked. The move also dragged down prices of gold, copper and even the euro.

"Traders were looking like deer in the headlights," said Peter Donovan, a floor trader at Vantage Trading on the New York Mercantile Exchange. "I called four different desks, and they all said, 'we don't ...