With regulators taking a keen look at market structure, this is not a good time for system glitches. Apparently, a system upgrade at the NYSE Euronext's Arca triggered a near-10 percent drop in the SPDR S&P 500 ETF Trust, managed by State Street.

The snafu involved 7.2 million shares in the closing auction on NYSE Arca. This is a massively popular ETF, and in the wake of the Flash Crash report--which dealt with ETFs--it certainly raises questions. The exchange says the issue has been resolved and that operations will return to normal.

The problem appears to be this: A scheduled software glitch inadvertently kicked off a closing auction at 4 p.m., 15 minutes early. As a result, the closing price of the SPDR was shown as $106.46, down from the opening level of $117.74. That would have meant a $7.9 billion loss for the ETF. All trades at the $106.46 price were later voided, and the closing price was updated to $118.54.

In the current environment, it raises the issue of pricing mishaps that can really escalate. Lots of money managers have voiced concern. The Twitter-sphere erupted. One commentator is calling it an "after-hours Flash Crash in the SPDR," which is not exactly comforting word choice.

For more:
- here's a MarketWatch article

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