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Greenberg: My Worst Mistakes

Tickers in this article: ISRG MNST NFLX SBUX

(This column previously appeared on RealMoney Pro )

SAN DIEGO ( TheStreet) -- Nobody's always right when it comes to stocks.

I've learned that the hard way more than a few times in decades of reporting, researching and writing about companies. Even the smartest -- yes, that includes Warren Buffett -- sometimes get it wrong.

In the end, when I mess up, I fess up -- and I always do a personal post-mortem. It's humbling, but it helps and often comes in handy as a reference point, and hopefully a lesson learned.

If there's any theme, it's this: The worst mistakes often come from being too smart for your own good, especially when putting too much trust in your experience, perspective and instincts. It's important to trust your instincts, and I think mine are very good when supported by what I believe to be solid research -- and often just common sense. But every now and then, they steer me wrong. ( Trust, but verify -- and verify, again!)

These are a few of my biggest blunders, which also turned into big stocks.

Raising red flags on Starbucks shortly after its initial public offering . The concept of premium coffee just seemed like a ridiculous idea. First, there was the question: Who in their right mind would pay a premium price for what many said was little more than burned coffee? (That was always one of the big knocks, back in the day.)

Perhaps more troublesome: Starbucks  opening up stores across the street from one another. In most retail and restaurants, that creates instant cannibalism. It simply doesn't work . But for Starbucks, not only did it work (as we all learned after the fact), it was genius. Starbucks had figured out traffic patterns: That even for something as addictive as coffee, people wouldn't cross the street for it. They'd simply go elsewhere.

Lesson: The rules of retail can indeed be broken.

Raising red flags over Monster Beverage, back when it was called Hansen Natural . I had covered the beverage industry earlier in my career, including Coca-Cola , PepsiCo  and a bunch of smaller brands. I made good industry contacts and got to know the industry. One rule of thumb I just assumed would apply this time: Never mess with the big guys. Monster Beverage had evolved out of a juice company, using guerilla marketing, to create what rapidly became not just a monster brand, but a monster of a stock. I just assumed that if the product really had legs, Coke would create its own energy drink. And with Coke's distribution, it would annihilate Monster (it tried but failed.) And if it didn't, I thought health concerns surrounding energy drinks and teens, even back then, would do the concept in. (Not yet!)