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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