This is what patiently aggressive investors live for -- a world-beating company whose stock falls 44% in less than three weeks while the market is flat (the Dow (INDEX: ^DJI) was down 0.2% in that time).

Did I mention that it's down on good news?

This isn't a history lesson. It just happened to this month's featured stock.

What happened
The trouble started when premium foam mattress maker Tempur-Pedic (NYSE: TPX) reported earnings on April 19. Its quarter was quite strong. It increased sales 18% over the 2011's first quarter and was operationally efficient enough to grow bottom-line earnings per share by 26%. That's strong growth on an absolute basis and was enough to beat analyst estimates.

So what gives?

Seasoned earnings watchers know that lowering guidance for the future can hose stock prices even if earnings are strong.

But that's not what Tempur-Pedic did. It kept the same full-year guidance it gave three months beforehand. The stock price fell because analysts were hoping for a guidance boost after a strong quarter.

Because management is being conservative and not micromanaging its guidance, the implied full-year growth is only 15% on the top line and 22% on the bottom line. Due to basic math, that's lower than its first-quarter performance.

Then on May 7, Tempur-Pedic confirmed that it was discounting one of its mattress lines 12% to 17% as a Memorial Day/Fourth of July promotion.

That combination of events scared the heck out of the market, resulting in a 44% stock price drop, because it seemed to confirm the big fear with Tempur-Pedic -- that increased competition from the likes of traditional innerspring mattress makers Sealy (NYSE: ZZ), Simmons, and Serta would drive down Tempur-Pedic's enviable margins. Note that each of the "Big S's" now has a competing memory foam product. That's a much bigger threat than the generic knockoffs that have hounded Tempur-Pedic for years.

The reality
Yes, Tempur-Pedic faces stiffer competition via imitation now because of its success. But that's true of any successful consumer brand, from Coke to Tide detergent.

And management knew it was coming. As Dale Williams, Tempur-Pedic's CFO since 2003, puts it:

We've said for as long as I have been with this business, the long-term goal here, the long-term gain is not to necessarily maintain our share of specialty. We want specialty to become gain share in the market. And we expect to lose some share in specialty along that way. But if specialty is 20% of the market, and we are 70% of specialty, that's nice. But we'd much rather be 50% or 45% of specialty, and specialty be 50% of the market. That's a much better equation for us.

In other words, Tempur-Pedic doesn't care if it has a smaller percentage of the specialty (aka non-inner-spring) mattress business as long as that pie keeps growing. It's showing that in its actions -- the same actions that are scaring the market.

It boosted its advertising spending by 37% to increase its brand recognition (and thwart imitators). It's giving retail partners better incentives in order to increase its distribution footprint and store presence. It's kept up its research and development spending to better meet customer needs. And it's attacking its attackers.

How so? Some of that R&D spending over the last two years was used to develop its new lower-priced Simplicity line. Tempur-Pedic has traditionally played at price points of $2,000 and above. Now the Simplicity line allows it to play in the $1,000 to $2,000 range, which effectively doubles its addressable market. Watch out Sealy, Simmons, and Serta. You too, Select Comfort (Nasdaq: SCSS).

Given the lower price points, the heavy spending on advertising and dealer incentives, and the continued R&D, you'd think margins would be getting crushed. You'd be wrong. The same guidance that the market thinks is weak calls for gross margins to increase 2% this year (they were a whopping 52.4% last year). And the net margins are robust enough that Tempur-Pedic now sells for just 12 times its earnings guidance for this year.

The bottom line 
Tempur-Pedic is one of my favorite companies, and I've personally owned its stock for a few years now. I've watched it disrupt the mattress industry with its high-end foam mattresses, and I expect it to continue to do so under its long-term-focused management. And I believe Tempur-Pedic's current share price weakness is a great buying opportunity. 

For another great real-time buying opportunity, read our free report: "The Stocks Only the Smartest Investors Are Buying." It details a sector of the market Warren Buffett invests in heavily but many individual investors avoid. Just click here to read more.

Anand Chokkavelu owns shares of Tempur-Pedic. The Motley Fool has a disclosure policy. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. Try any of our Foolish newsletter services free for 30 days.