The New York Stock Exchange is seen during morning trading on January 19, 2023 in New York City.
New York CNN  — 

The New York Stock Exchange experienced a technical issue Tuesday morning that led to a brief trading halt for dozens of major companies just after the market opened. As a result of the issue, some trades that occurred before the halt will be made “null and void,” according to an exchange representative.

Overall, more than 250 stocks were impacted, including such major names as Verizon, McDonald’s, Morgan Stanley, AT&T, Nike, Mastercard, Uber, Wells Fargo, Shell, 3M, Sony, UPS, Visa, Walmart and Exxon Mobil, according to the NYSE.

Many of those stocks made large moves just minutes into the morning trading session, sending the shares of companies like Wells Fargo and Morgan Stanley into a nosedive.

Morgan Stanley briefly plunged to $84.93 after ending at $97.13 on Monday before recovering. McDonald’s and Walmart also fell more than 12% before trading was halted.

By 9:50 a.m. ET Tuesday all affected companies on the NYSE had resumed trading, according to a status report from the NYSE that said “all systems are currently operational.”

What happened

Stocks typically open for trading on the NYSE at 9:30 a.m. ET, and each stock is given an “opening price” that is determined by the thousands of orders that accumulated overnight and early in the morning ahead of the opening bell. The exchange compiles these buy and sell orders and formats a single price. That price is then quoted at the open and is known as an “auction print.”

In an emailed statement, exchange officials said opening auctions “did not occur” for a number of these stocks, after a “system issue” prevented the accumulated orders from being compiled into the opening price of some stocks on Tuesday.

That meant those shares opened with supply-demand imbalances at prices very far from where they closed on Monday.

The exchange said that the trades made before an opening price was printed will be reviewed as “clearly erroneous” under their rules and could be declared null and void.

The NYSE did not explain the cause of the technical error. The US Securities and Exchange Commission also said Tuesday it was reviewing the issue.

Prior halts

NYSE, and most other major stock exchanges, issue automatic halts for stocks that move dramatically up and down.

In May 2010, the Dow plunged during a “flash crash,” before dramatically rebounding. A report by US regulators later said the huge swing was the result of high-frequency trading activity following a massive trade by a single market participant. That sparked a number of regulatory changes aimed at safeguarding the equity markets, including a “limit up-limit down” mechanism that prevents trades in individual stocks from swinging outside a specific price band.

But exchange officials can also halt trading when there is a technical problem. Last fall, three Canadian stock exchanges suffered a 40-minute outage due to a “connection issue,” before the exchange was able to isolate and repair the problem.