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 optical networking equipment maker Finisar (Nasdaq: FNSR) plummeted more than 10% in intraday trading today, just one week after marking a new 52-week high.

So what: There's high volume in the sell-off, which suggests profit taking by large institutional owners of the stock. Wellington Management, administrator of the Vanguard family of funds, and BlackRock (NYSE: BLK) were the two largest outside shareholders as of June 30.

Now what: Without any business cause for a sell-off, I'm loath to call this anything other than Mr. Market acting like, well, Mr. Market. Brocade Communications (Nasdaq: BRCD), a customer, just recognized Finisar as one of its top suppliers, and the stock was trading for a reasonable 15.5 times next year's earnings before the drop. If you're a long-term investor looking for a buying opportunity, panicked traders just handed you one.

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Fool contributor Tim Beyers is a member of the Motley Fool Rule Breakers stock-picking team. He didn't own shares in any of the companies mentioned in this article at the time of publication. Check out Tim's portfolio holdings and Foolish writings, or connect with him on Twitter as @milehighfool. You can also get his insights delivered directly to your RSS reader. The Motley Fool is also on Twitter as @TheMotleyFool. Its disclosure policy is at least 10% better than other disclosure policies.