The latest example came Wednesday night when the nation’s third-largest stock exchange operator, BATS Global Markets, alerted its customers that a programming mistake had caused about 435,000 trades to be executed at the wrong price over the last four years, costing traders $420,000.
A day earlier, the trading software used by the National Stock Exchange stopped functioning properly for nearly an hour, forcing other exchanges to divert trades around it. The New York Stock Exchange, the nation’s largest exchange, has had two similar, though shorter-lived, breakdowns since Christmas and two separate problems with its data reporting system. And traders were left in the dark on Jan. 3 after the reporting system for stocks listed on the Nasdaq exchange, the second-biggest exchange, broke down for nearly 15 minutes.
The stream of errors has occurred despite the spotlight on the exchanges since a programming mishap nearly derailed Facebook’s initial public offering on Nasdaq last May and BATS’s fumbling of its own I.P.O. two months earlier. At the end of 2012, a number of exchange executives said they were increasing efforts to reduce the problems. But market data expert Eric Hunsader said that the technology problems have become, if anything, more frequent in recent weeks.
Matt Samelson, the founder of the industry consultancy Woodbine Associates, said, “Now that the world is watching, everyone is trying to be more rigorous. Their increased rigor is not yielding the benefits they hoped.”
Joe Ratterman, the chief executive of BATS, said Thursday that he viewed the firm’s announcement this week as a sign of markets that were functioning well, given his firm’s ability to find a problem that he called an “extreme edge-case scenario.”
“We discovered this problem and reported it — it’s a positive thing,” Mr. Ratterman said. “It’s being covered as if it’s a negative issue, and a continuation of a series of problems.
“Call me an optimist, but I see positive indications of the markets moving forward,” he said.
Regulators and traders have said that malfunctions are inevitable in any complex computer system. But many of these same people say that such problems were less frequent before the nation’s stock exchanges were thrown into a technological arms race in the middle of the last decade as a host of upstart exchanges like BATS challenged incumbents like the New York Stock Exchange.
The nation’s 13 public stock exchanges now compete fiercely to offer the latest, fastest and most sophisticated trading software, in part to appeal to the high-speed trading firms that have come to account for over half of all stock trading. With each tweak comes a new opportunity for a mistake to be inserted into the system.
“The rate of change is getting so rapid that the quality assurance process isn’t as robust as it should be,” said George Simon, a partner at Foley & Lardner who used to work at the Securities and Exchange Commission, which oversees the nation’s stock markets. “This has been something that has been brewing now for five years, and it keeps getting worse.”
Mr. Simon said that in less fragmented and complex markets, technology problems had been less common.
The market malfunctions have been assigned part of the blame for the diminishing amount of trading happening on the nation’s stock exchanges. The total volume of daily trading was down 17.6 percent in 2012 from 2011, according to Rosenblatt Securities.
Mr. Samelson of Woodbine Associates said the problems had long rattled retail investors, but they were becoming increasingly worrying for big institutional investors as well. While he was talking about the BATS mishap on Thursday, he received a text message from one big investor who said, “as if we didn’t have enough bad news.”
The problem reported by BATS was different from many other recent problems because it did not halt trading. Instead, the programming error meant some trades were not executed at the best price, as exchanges are required to do by law.
Only a small category of very complex trades were executed at the wrong prices, all of them coming from investors trying to do a so-called short sale of stocks. The 435,000 erroneous trades were only 0.003 percent of all trades over the last four years, according to Mr. Ratterman.
“This is so hard to identify that no customer ever identified it,” Mr. Ratterman said.
Mr. Ratterman said that 119 member firms lost money. He said he was not yet sure if BATS would compensate its members for their losses. BATS informed the members and the S.E.C. of the problem on Wednesday night, after discovering it on Friday.
The S.E.C. was not previously aware of the problem, but the enforcement division is already reviewing the issue, according to people with knowledge of the review who spoke on the condition of anonymity.
S.E.C. officials have acknowledged that they do not have adequate tools to properly police the high-speed, highly fragmented stock markets. But the agency has started several initiatives to catch up. Last year, the agency purchased software from a high-frequency trading firm that will give regulators a real-time window into the markets.
The agency has also been considering a rule that would force exchanges to submit their technology for regulatory review, something that some exchanges currently do voluntarily. At recent hearings called to examine the automation of the markets, members of the industry have supported other reforms to strengthen the system, like kill switches that would automatically stop errant trading.
Mr. Ratterman said regulators could make small changes to rules that would simplify the market infrastructure and make it less prone to mishaps.
But executives at some other exchanges have said that more sweeping changes are necessary. At a hearing in December, Joe Mecane, an executive at the New York Stock Exchange’s parent company, said that “technology and our market structure have created unnecessary complexity and mistrust of markets.”
Amy Butte Liebowitz, the former chief financial officer at the exchange, said that “you are only going to see more and more of this until someone says, ‘I’m not going to put up with this level of errors.’ ”
