Why a Whole Bunch of Stocks Popped and Plunged

Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of a wide variety of stocks were jumping and dropping more than 10% today after a trading algorithm at Knight Capital Group (NYSE: KCG  ) went badly awry.

So what: In all, the NYSE Euronext was digging into 148 tickers of stocks and ETFs potentially affected by the erroneous trading. The full list of stocks affected included smaller, easily moved stocks like Triangle Petroleum (Nasdaq: TPLM  ) and Magnum Hunter Resources (NYSE: MHR  ) as well as giants like General Electric (NYSE: GE  ) and Pfizer.

What should investors make of these moves? For the vast majority, not much. There are some companies -- Harley-Davidson (NYSE: HOG  ) , for instance -- that happened to also release news today. However, it's easy to pick apart the impact of news versus the trading issue. If the big move in the stock took place early in the day and quickly reversed itself, then the move you saw is most likely the impact of the trading malfunction, not the news release.

Sticking with Harley as our example, the company reported earnings today and beat on the bottom line but missed on sales. The stock is trading down a bit, but the big, 10%-plus move of the day came early in trading and seems very obviously the result of the Knight Capital trading.

Now what: For the vast majority of the 148 stocks involved here, the "now what" is for investors to just breathe and move on. As far as I'm concerned, it's just not that big of a deal.

Who is this a big deal for? Well, Knight Capital. Not only is this a big black eye for the firm, but it could also be a costly error. That view is showing up in the company's stock, which lost a full third of its value today.

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Motley Fool newsletter services have recommended buying shares of Pfizer and NYSE Euronext. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.

Fool contributor Matt Koppenheffer has no financial interest in any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter, @KoppTheFool, or on Facebook. The Fool's disclosure policy prefers dividends over a sharp stick in the eye.


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