Although Warren Buffett is revered as one of the greatest investing minds of his time, that doesn't mean he hasn't had some missteps or regrets along his six-decade-plus journey.
It's one thing to make an investment and realize you made a bad choice; it's another thing when you've watched a stock flourish while you watch from the sidelines, wishing you'd bought it. When it comes to the latter with Buffett, Microsoft (MSFT 0.51%) fits that description.
Image source: The Motley Fool.
Buffett has openly been a fan of Microsoft's business for many years, but has publicly said that his close friendship with its co-founder Bill Gates kept him and Berkshire Hathaway from investing in the company. At Berkshire's 2018 annual meeting, Buffett said: "It just would be a mistake for Berkshire to buy Microsoft."
Given Buffett and Gates' relationship, Berkshire investing in Microsoft could seem like favoritism and a conflict of interest. It could also run the risk of people (especially regulators) questioning whether Buffett has access to inside information not publicly available.

NASDAQ: MSFT
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It's unfortunate that Buffett and Berkshire missed the train, as Microsoft has been one of the best-performing investments the market has seen. It also checks many of the boxes Buffett looks for in a company. It's now valued at over $3.5 trillion (up by over 780% in the past decade), and is currently the fourth-most-profitable public company in the world.