By 2001, MSFT had 18 million shares of Apple, sold by 2003. Later a 2:1 and then a 7:1 stock split, would give them a value of $36 billion today (if my math and the article's math are correct), but was sold for a relatively small profit instead.
It's harder than you think to hold onto a massive amount of shares in a growing company.
There are lots more, here's a more systematic study from Fidelity:
"
The average investor lost money in the Fidelity Magellan fund under Peter Lynch’s tenure during a period of time when the fund returned around 29% annually."
It's easy to look back and say, wow, if I had invested at that time, I'd have $x m/b, but the temptation to "lock in gains" or "diversify" is really strong. And sometimes it is right!