The US sharemarket showed again on Thursday that it remained vulnerable to technological breakdowns even as regulators and market operators work to keep up with trading that is increasingly electronic and driven by speed.
The latest trouble shut down trading on the Nasdaq market and its more than 3000 stocks for more than three hours on Thursday afternoon.
The disruption on the nation's second-largest stock market reverberated around Wall Street, affecting other markets as investors cautiously stepped back. Brokers scrambling to trade elsewhere discovered they could not complete trades while in the dark about prices on Nasdaq.
"It is everybody - nobody can trade," Manoj Narang, chief executive of Tradeworx, said during the afternoon. "I've never seen anything like this."
Nasdaq officials said the halt was prompted by a problem with the data system that disseminates prices, and that its cause had been "identified and addressed".
The breakdown came just two days after Goldman Sachs accidentally sent out a barrage of errant trades that disrupted options exchanges.
The two episodes have amplified questions about the reliability and integrity of financial markets that companies depend on to raise money and Americans trust with their retirement savings.
While regulators and market participants have taken several steps to strengthen their systems, the problems this week suggest that the flaws have not been repaired and may be getting worse.
The persistence of technical flaws - each seemingly coming from a different part of the system - has been blamed on the complexity of the trading technology and the fragmentation of the market itself. In contrast to the days when the New York Stock Exchange competed only with the Nasdaq, today there are 13 public exchanges competing in a fast-changing and low-margin business.
The frustrations over the breakdowns represent an immediate challenge for the regulatory agency that oversees sharemarkets, the Securities and Exchange Commission, and its new chairwoman, Mary Jo White.
Ms White said the paralysis at Nasdaq was "serious and should reinforce our collective commitment to addressing technological vulnerabilities of exchanges and other market participants".
She said she would push ahead with rules that would add testing requirements and safeguards for trading software. So far, those rules have faced resistance from the exchange operators.
Despite the turmoil, stock prices were unscathed. The Nasdaq rose slightly after trading resumed and finished the day up 1.1 per cent.