There were some pretty surprising winners last month. Bucking the "sell in May" mantra, these unlikely companies roared to life. Investors didn't mind, of course. Let's go over a couple of the stocks that put the pedal to the metal.

MannKind (NASDAQ:56400P706) -- Up 36%
Investors have always believed that July would be the big month for MannKind. After all, that's when when regulators are likely to approve Afrezza, MannKind's potentially breakthrough inhaled insulin treatment.

However, the "buy on the rumor, sell on the news" mindset appears to be kicking in early. Piper Jaffray upgraded the stock in May, encouraged by both the likelihood of approval and its chances of lining up a big-name distributor to push Afrezza. Given the stock's surge, it wouldn't be a surprise if MannKind slips this summer even if the news is positive.    

Netflix (NASDAQ:NFLX) -- Up 30%
The leading premium video service had a spectacular month of May. The stock rallied after it made a price increase official, boosting its monthly rate to $8.99 for new subscribers. It also announced expansion into Continental Europe.

Investors didn't even flinch when its closest yet distant rival locked up lucrative HBO content in late April that debuted it in mid-May. Netflix had corrected sharply since peaking in March, and the bounce was worth the wait.  

Build-A-Bear Workshop (NYSE:BBW) -- Up 25%
The build-it-yourself teddy bear chain that was all the rave for kids' parties a decade ago isn't exactly back. Build-A-Bear is still struggling with comps and closing down stores.

However, the company clearly did something right in its latest quarter, with gross margins expanding and profitability exploding to $0.29 a share after merely breaking even a year earlier. Analysts were only expecting net income of $0.11 a share. This is the first time in a year that Build-A-Bear hadn't disappointed on the bottom line, and it came through in a major way.

Build-A-Bear is down to just 316 company-owned stores, but its store-level performance is showing signs of stabilizing. With its debt-free and cash-rich balance sheet there may be signs of life here after all.   

Keurig Green Mountain (UNKNOWN:GMCR.DL) -- Up 20%
It's been a good year for the company formerly known as Green Mountain Coffee Roasters. The leader of single-serve blasts of java has seen its stock soar 49% so far in 2014, and a good chunk of that came in May.

The largest catalyst for Keurig is that Coca-Cola (NYSE:KO) moved to initiate a 10% stake in the coffee company earlier in the year, and in May announced its intention to increase its position to 16%. Having the world's largest beverage company as an investor has its benefits, and that's even before Keurig enters the carbonated market, with the launch of Keurig Cold expected in its next fiscal year. 

It also didn't hurt that Keurig kicked off the month by posting another blowout quarter with better than expected earnings and a $1 billion stock buyback authorization. That's one way to boost already caffeinated shares.