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 online restaurant reservation specialist OpenTable (Nasdaq: OPEN) had a case of stock-market food poisoning, regurgitating as much as 12% of its price after a cautionary analyst report.

So what: Barclays analyst Mark May throttled back his optimism on OpenTable today, dropping his rating on the stock from Overweight to Equal Weight -- or, essentially, from buy to hold. May cited slowing growth as a prime concern and projected that June-quarter revenue would be lower than the March-quarter tally. May also cut his share-price estimate for OpenTable from $50 to $43.

Now what: For investors who already own OpenTable or have been sniffing around the stock, May's report is a good reason to take a closer look and see whether there really is reason to be concerned about the company's growth. After all, with shares trading at about 32 times 2011 earnings, investors need to be able to count on strong, steady growth to have the investment pay off. That said, May's is just a single view on OpenTable, so it's probably not advisable to make your sell decision based solely on this one report.

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Fool contributor Matt Koppenheffer owns shares of Barclays but has financial interest in any of the other 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.