NYSE market mania 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. So what happened? We're not yet sure of the why (the US Securities and Exchange Commission is investigating the issue) but we do know the what.  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.