"Mysterious Algorithm Was 4% of Trading Activity Last Week"

CNBC reports (h/t Instapundit):

A single mysterious computer program that placed orders — and then subsequently canceled them — made up 4 percent of all quote traffic in the U.S. stock market last week, according to the top tracker of high-frequency trading activity. The motive of the algorithm is still unclear.

The program placed orders in 25-millisecond bursts involving about 500 stocks, according to Nanex, a market data firm. The algorithm never executed a single trade, and it abruptly ended at about 10:30 a.m. Friday.

“Just goes to show you how just one person can have such an outsized impact on the market,” said Eric Hunsader, head of Nanex and the No. 1 detector of trading anomalies watching Wall Street today. “Exchanges are just not monitoring it.”

. . . the ultimate goal of many of these programs is to gum up the system so it slows down the quote feed to others and allows the computer traders (with their co-located servers at the exchanges) to gain a money-making arbitrage opportunity.

. . . Hunsader warned that regulators better do something fast, speculating that this single program could have led to something very bad if big news broke or a sell-off occurred and one entity was hogging this much of the system.