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 visual data analysis expert Cognex (Nasdaq: CGNX) took their eye off the ball today, falling as much as 10.9% on heavy trading.

So what: Analyst firm Piper Jaffray just downgraded Cognex from buy to hold, citing high risk and an overheated valuation. Much of Cognex's sales come from the semiconductor industry, and after scary reports from Intel (Nasdaq: INTC) and others, the prospects for great sales into that flagging sector appear "weak."

Now what: Cognex did indeed look a bit overvalued heading into this downgrade, share prices having risen 83% over the last year to leave rivals such as AMETEK (NYSE: AME), Orbotech (Nasdaq: ORBK), and MKS Instruments (Nasdaq: MKSI) firmly in the dust. The stock remains richly valued, but then it wasn't cheap when the Stock Advisor team picked it out for our flagship newsletter, either. Jaffray's concerns seem shortsighted to me, making this a buy-in opportunity for patient investors. Learn more about Cognex with a free 30-day trial of Stock Advisor.

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