An overly violent opening of trading in some stocks listed on the New York Stock Exchange caused panic on Wall Street on Tuesday as dozens of major U.S. companies lost billions of dollars in market value for no apparent reason, Bloomberg writes.

Trading in dozens of large-cap stocks was halted within the first 30 seconds of the start of Tuesday’s session after they showed huge volatility, baffling investors. Transactions on the New York Stock Exchange returned to normal less than 20 minutes later.

As a result, Wells Fargo & Co (NYSE:WFC) shares collapsed 15%, Walmart (NYSE:WMT) lost $46 billion, and AT&T Inc (NYSE:T) seemed to oscillate between rising 20% and falling 21% in a matter of seconds.

Ken Mahoney, CEO of Mahoney Asset Management, said he tried to sell shares of companies like AT&T Inc. and Exxon Mobil Corp (NYSE:XOM). back in early trading. For now, the NYSE is investigating the events at the opening of trading and finding out the full circumstances. Investment and trading firms may consider filing claims for trades affected by the glitch. The SEC is also looking into the trading activity

However, investors soon realized that this was a more serious problem. Although details of what happened are scarce at this point, experienced traders advise avoiding orders at the open.

Computer glitches that lead to volatile pricing and affect trading are not uncommon on U.S. exchanges. Perhaps the most famous glitch occurred in August 2012, when faulty software used by one of the largest firms, Knight Trading, flooded the exchanges with erroneous orders and caused market swings. And last year, Citigroup Inc (NYSE:C)’s London trading desk was behind a sudden crash that sent stocks tumbling across Europe.

The problem most likely affected intraday traders and those who use algorithmic trading, but not long-term investors.