Microsoft's co-founder Paul Allen owned a huge portion of AOL stock back before AOL got huge.
But then he sold it all in the 1990s.
What did that stake ended up being worth?
$40 billion.
Whoops.
This nugget comes from a Times review of Allen's autobiography:
There’s a refreshing candor to the prose, too. Allen spends the better part of one chapter demonstrating how a meddlesome owner can foul up a basketball franchise; in another, he shows that being too smart by half can mess up a professional football team. He confesses to losing $8 billion investing in cable companies in the 2000s. And then there was his decision to sell the large stake he had amassed in AOL in the early 1990s. He kicks himself for letting Gates convince him that a pipsqueak like AOL stood no chance against Microsoft, but he also shares the colossal size of his blunder: selling, it turned out, was a $40 billion mistake.
Don't feel too bad. Allen has billions of billions of dollars.
Correction: An earlier version of this post reported that Bill Gates convinced Paul Allen never to buy AOL stock, and that this was the $40 billion mistake. The $40 billion mistake was actually Allen's decision to sell AOL stock too soon.
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