Trading in Citi was halted at 1:03 pm EDT, after a trade of 8,820 shares at $3.3174, a 12.7% drop from the previous trade, according to Dow Jones.

After a five minute pause, trading resumed with the stock down about 5.5%. The trade was later cancelled.

The circuit breaker rules came into effect two weeks ago, requiring trading in a Standard & Poor's 500 stock to pause for five minutes if its price changes by more than 10%.

The rules - which are initially running for a six month pilot - were introduced in response to the so-called "flash crash" on 6 May, which saw the Dow Jones industrial average plummet in minutes, with 30 stocks in the S&P falling at least 10%.

The circuit breaker was first called into action on 16 June, when the Washington Post Company's share price nearly doubled after an erroneous trade.

Patrick Healy, from the Issuer Advisory Group told the New York Times that the Citi incident shows the new system is working and would have prevented the chaos seen last month.