HFTs baby. HFTs. They just provide liquidity.

Some more details from Dow Jones on the latest mini daily flash crash.

Shares of Citigroup Inc. (C) fell as much as 17% Tuesday before trading was halted in the stock for five minutes as a result of the new circuit-breaker pilot program.



The trading halt was only the second time that a single-stock circuit breaker has been hit since a pilot program started earlier this month. It was the first to be triggered for a slide. The first trading halt from the program was prompted by a surge in shares of the Washington Post Co. (WPO) on June 16 that was later deemed erroneous.



The halt in Citi's stock was triggered at 1:03 p.m. EDT, according to NYSE Euronext spokesman Raymond Pellecchia Jr. Pellecchia said it remains unclear whether the trade in Citi was legitimate or erroneous.



After the five-minute pause, trading in Citi resumed with the stock down about 5.5%.



The halt came after a single trade was placed for 8,820 Citi shares at $3.3174, a 12.7% drop from the previous trade, where Citigroup shares were priced at $3.8. The $3.3174 price represented a 17% drop from Monday's closing price.



Pellecchia said the $3.3174 quote appeared on the Alternative Display Facility run by FINRA. The ADF is a facility for posting quotes, and reporting and comparing trades.



FINRA wasn't immediately able to confirm where the trade occurred and whether it was erroneous.



The sharp drop in Citigroup caught much more attention than Washington Post's June 16 surge, because it is much more widely held. Citigroup's market capitalization stands at around $109 billion, far greater than Washington Post's market cap of just over

$3 billion.



"Obviously once the trade hit the tape, it set off alarm bells all over the place," said Craig Peckham, equity trading strategist at Jefferies & Co.



Still, he added, "the pause in the market gave people time to digest what happened and the market impact appears to be relatively neutral."



Earlier this month, the exchanges launched the initial circuit breakers tied to individual stocks within the S&P 500 as a safeguard against the type of rapid plunges the market experienced on May 6. The market-wide circuit breakers establish a five-minute trading halt if a stock moves by 10% or more in the preceding five minutes.