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 Green Dot Corp. (Nasdaq: GDOT) fell nearly 15% on heavy volume in early trading after the company reported lower-than-expected fourth-quarter revenue and an analyst advised investors to sell.

So what: There's no doubting Green Dot's growth. The company, which supplies prepaid debit cards to lower-income consumers, saw revenue rise 32% to $91.9 million. Trouble is, analysts were expecting $96.8 million.

Now what: At first blush, Green Dot seems like an obvious winner. Wal-Mart (NYSE: WMT) is an investor, and the company is part of a U.S. Treasury pilot program in which some consumers will get federal tax refunds via Green Dot's debit cards, Reuters reports.

Janney Capital Markets analyst Thomas McCrohan isn't convinced. He thinks the stock's valuation doesn't account for the risk of increased competition. "While we like the business model and prospects for reloadable prepaid cards, our rating reflects our assessment of fair value," McCrohan said in an interview with The Associated Press.

McCrohan's concerns are well-taken. Also, with the stock trading for roughly twice the long-term growth rate analysts expect even after today's sell-off, I suspect Fools need not rush to open a position.

Interested in more info on Green Dot? Add it to your watchlist.

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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 Twitter as @milehighfool. You can also get his insights delivered directly to your RSS reader. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool owns shares of Wal-Mart Stores and is also on Twitter as @TheMotleyFool. Its disclosure policy is at least 10% better than other disclosure policies.