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In 2008, nearly everybody made bad investments. By comparison, 2009 was a complete delight.

Still, investors made mistakes. Even with the huge rally, some stocks, including Valero Energy (NYSE:VLO), MetroPCS (NYSE:PCS), and Apollo (NASDAQ:APOL), managed to drop this year. In addition, Treasury bonds, which rode high during 2008, also knocked investors for a big loss this year.

What might have been
Personally, though, my biggest mistake this year was in cutting my winners short. The decision to take some profits on part of my positions in Starbucks (NASDAQ:SBUX) and Freeport-McMoRan (NYSE:FCX) well into the rally seemed like a no-brainer at the time, but those stocks just kept going up. Similarly, I trimmed my holdings in junk bond mutual funds and some international bonds, figuring that gains might well prove short-lived. Although that didn't cause any losses, the opportunity cost of getting out too early was extremely high.

There's a lesson in that. Plenty of investors are smart enough to buy top performers like Apple (NASDAQ:AAPL) and Green Mountain Coffee Roasters (NASDAQ:GMCR), and a good number of them earn decent profits from them. But only the most disciplined, patient investors stick with those winners for the long haul, squeezing every penny of potential gain out of their shares and turning a modest winner into a gold mine for their portfolio.

How about you?
But we want to hear from you, and we're willing to sweeten the pot to make it interesting. So here's the pitch: Leave us a comment below telling us about your worst investing mistake this year and the lesson you learned from it, in 250 words or fewer. Do it before 8 p.m. ET on Thursday, Dec. 17, and you'll automatically be entered into our Worst Investing Mistake Contest. Our editorial team will select a winner from the comments we receive.

If you win, we won't give you back any money you lost on bad investments -- sorry. But we will give you a one-year digital subscription to our Motley Fool Income Investor service (a $199 value), which will give you full access to winning advice on income-generating investments like dividend stocks. With lead advisor James Early and his team on your side, you'll make a lot fewer mistakes in 2010. Read the complete list of contest rules right here.

Mistakes are only bad if you don't learn from them, so don't hesitate -- tell us yours today!

Fool contributor Dan Caplinger has made his share of mistakes. He still owns some shares of Starbucks and Freeport-McMoRan, though not as many as he'd like. Green Mountain Coffee Roasters is a Motley Fool Rule Breakers recommendation. Apple and Starbucks are Stock Advisor selections. Apollo Group is an Inside Value recommendation. Good or bad, the Fool's disclosure policy tells all.