NEW YORK ( TheStreet) -- It's easy for investors to share their greatest stock trades, but somewhat difficult to admit the big mistakes. Frankly, I learn much more from the mistakes than from the triumphs, but there are sometimes lessons in those, too. Most investors will have several of both, and can build on their knowledge with each success and failure.A well-known fund manager once told me that he had a board outside his office plastered with the stock certificates of the companies he'd taken positions in that turned out to be mistakes. Rather than burying his worst ideas, this manager was reminded of them each day as he entered his office. That, to me, is a brilliant way to reinforce what you have learned from the ideas that went bad. With that said, I'd rather start this column off with a mistake. It's a fairly recent example, and the company in question is Green Mountain Coffee Roasters ( GMCR), purveyor of Keurig machines and the infamous single brew K-cups that have taken the hot beverage industry by storm over the past several years. What a stupid, expensive idea, I thought, especially as the stock rose higher and higher, and the multiples soared to ridiculous heights. Fifty cents or so for a cup of coffee in my own home seemed a bit crazy to me, when I could brew a whole pot for the same amount or less.