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 QuinStreet (Nasdaq: QNST), which provides online marketing services, fell more than 19% in early trading on more than three times the average volume after the company reported disappointing first-quarter results and trimmed its revenue outlook for the full year.

So what: Q1 earnings beat expectations by a penny, but revenue fell 2% year over year to $101.2 million, well short of the $107.1 million analysts were calling for, according to data compiled by Yahoo! Finance. The miss was at least partly responsible for a Credit Suisse analyst's downgrade the stock from "outperform" to "neutral."

Now what: But if the miss was troubling, the outlook was catastrophic. QuinStreet now expects just $403 million in fiscal 2012 revenue, down sharply from summer estimates of $455 million to $475 million in sales, The Associated Press reports. Does the lower guidance worry you? Or would you buy shares of QuinStreet at current prices? Please weigh in using the comments box below.

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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 Google+ or Twitter, where he goes by @milehighfool. You can also get his insights delivered directly to your RSS reader.

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