What happened

Shares of data security expert Fortinet (NASDAQ:FTNT) closed 13.3% lower on Friday, following the release of strong third-quarter results with a side of bullish guidance. Earlier in the day, Fortinet's stock dipped as much as 14.5% lower.

Those are facts, not typos.

Two young office workers express disbelief at something on a shared computer screen.

Image source: Getty Images.

So what

In the third quarter, Fortinet's top-line sales rose 21% year over year to land at $454 million. Analysts would have settled for $451 million. On the bottom line, earnings jumped 75% higher to $0.49 per share. The consensus here stopped at $0.42 per share.

Looking ahead, Fortinet's management set their fourth-quarter guidance targets just ahead of the current analyst views. Revenue should grow roughly 19% over the year-ago period, landing near $495 million. Earnings are expected to come in at $0.51 per share, a 60% year-over-year gain.

And yet, Fortinet's shares plunged 13% lower.

Now what

The key to this sharp drop on a report packed with good news lies in the rearview mirror. Even now, Fortinet shares have more than doubled in 52 weeks and gained 14% over the last three months. These are market-crushing returns, leading to price-to-earnings and price-to-cash-flow ratios in Wall Street's nosebleed section.

I don't know what it would have taken to support Fortinet's shares at, say, Thursday's prices. The actual results just weren't impressive enough to keep the good times rolling. That being said, security is a red-hot sector right now, and Fortinet is positioned for long-term success, particularly in the even hotter cloud security market.

Expect more volatility in the months and years ahead. This time, we got an unexpected drop.

Anders Bylund has no position in any of the stocks mentioned. The Motley Fool recommends Fortinet. The Motley Fool has a disclosure policy.