Authorities spotted what appears to be a $7.9 billion computer glitch at NYSE Euronext's Arca stock market.

A system upgrade at Arca triggered an apparent 9.6% drop in an exchange-traded fund Monday, Bloomberg News said. That plunge would have knocked $7.9 billion off one of the country's most popular securities.

The SPDR S&P 500 ETF Trust was trading at $106.46 at 4.15pm Monday, compared with an opening level of $117.74.

These prices were then voided and the closing price updated to $118.54.

"Something went wrong, the question is why," James Angel, a finance professor at Georgetown University in Washington, told Bloomberg News. "For them to cancel the trades is probably causing heartburn in back offices across the country."

The ETF, which tracks the Standard and Poor's 500 Index, has a market value of more than $80 billion.

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