Glance at a five-year chart for Green Mountain Coffee Roasters (Nasdaq: GMCR), and you'll see why it's been an awe-inspiring performer for longtime shareholders. However, this week's quarterly results stirred up fears of weak and bitter brews in the coffee company's future.

Second-quarter net income surged 90% to $24.7 million, or $0.54 per share. Revenue rocketed 68% higher, to $324.9 million. Green Mountain was also able to improve gross margin to 33.4%, from 32.1% a year ago.

In more great news, the company said K-Cup sales were up 92%, while demand for its single-serve Keurig brewers is still going strong. It shipped 731,000 of the brewers, compared to 486,000 this time last year.

Green Mountain has been a highly caffeinated stock in recent years. Despite competition from usual suspects such as Starbucks (Nasdaq: SBUX), Peet's (Nasdaq: PEET), and Caribou (Nasdaq: CBOU), and even the unusual likes of Dunkin' Donuts and McDonald's (NYSE: MCD), Green Mountain has generated torrid sales and profit, as well as a surging stock price. Its stock is up 120% in 12 months.

Green Mountain also declared a 3-for-1 stock split. Savvy Fools know that stock splits don't really signify much, beyond creating a psychological impression that a stock is cheap.

But despite this quarter's aromatic earnings, Green Mountain's weaker-than-expected outlook left investors with a bitter taste in their mouths. The company said it now expects third-quarter earnings of $0.50 to $0.54 per share, well below analysts' estimates of $0.57. (Those figures are pre-split, by the way.)

Green Mountain's sliding stock price isn't too surprising, given the lackluster news. Shares are down about 26% from its 52-week high. As Buffalo Wild Wings' (Nasdaq: BWLD) recent precipitous stumble illustrates, premium-priced stocks tend to plunge on any whiff of disappointment. Green Mountain has long been a well-performing but premium-priced stock. With a price-to-earnings ratio of 49, it makes Starbucks look cheap at 26 times trailing earnings.

There's no reason for longtime shareholders to flip out and sell, but I wouldn't want to buy shares now, either. Even with the retreat in its price, I still say "buyer beware" on Green Mountain. It'd have to be a lot more reasonably priced to please my palate.

Green Mountain Coffee Roasters is a Motley Fool Rule Breakers selection. Starbucks is a Stock Advisor pick. Buffalo Wild Wings is a Motley Fool Hidden Gems recommendation. Try any of our Foolish newsletters free for 30 days.

Alyce Lomax owns shares of Starbucks. The Fool has a disclosure policy.