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 machine vision expert Cognex (Nasdaq: CGNX) were seeing double after getting whacked by Mr. Market and falling as much as 11% in intraday trading.

So what: Apparently it's a tough crowd trading Cognex. The fourth quarter was quite a good-looking period for the company. Revenue soared 66% year over year to $84.9 million, and earnings per share jumped from just $0.01 last year to $0.47. Both revenue and earnings topped Wall Street's estimates. The company's gross margin was down from the prior quarter, but at 72%, it was above the 69% level of 2009's fourth quarter. The company also said it set a new record for annual bookings.

Now what: So why the sell-off? Investors may be taking first-quarter guidance as a sign that the torrid growth is slowing back down. Revenue is expected to be in a range of $70 million to $73 million in the first quarter, which is down from the fourth quarter's tally but typical of the company's seasonality. However, Cognex will have to hit the top end of that range in order to hop the bar that analysts have set for the quarter.

With shares currently trading at 21 times expected 2011 earnings per share, investors are obviously expecting a lot out of the company, and it leaves Cognex with little room for error when it comes to keeping the growth engine cranked up. That said, today's sell-off looks like a bit of an overreaction.

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Fool contributor Matt Koppenheffer does not own shares of 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 his RSS feed. The Fool’s disclosure policy prefers dividends over a sharp stick in the eye.