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What: Shares of Qualys (NASDAQ:QLYS) plunged by as much as 20% after the company reported earnings and offered disappointing guidance.

So what: Revenue during the fourth quarter came in at $24.7 million, with the 19% increase over last year being driven by increased subscription sales and cross-selling to existing customers. Non-GAAP net income was $2 million, or $0.06 per share. Both figures bested analyst forecasts, which called for $24.5 million in revenue and $0.05 per share in profit.

Now what: The current quarter's profit guidance left something to be desired, though. Qualys is expecting revenue in the range of $24.4 million to $24.9 million, while investors were expecting $25.3 million in sales in the first quarter. On a non-GAAP basis, the bottom line should be between a loss of $0.01 per share to a profit of $0.01 per share, which is short of the $0.04 profit investors were expecting. Full-year sales should be in the range of $106 million to $108 million.

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Fool contributor Evan Niu, CFA, and The Motley Fool have no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.