Pooh, Back in early 2000, a fellow finance academic colleague of mine had 100% of his 401k funds from the previous 3 years invested in a NASDAQ heavy managed portfolio. Over the next year, his 401k portfolio fell by over 70%. Meanwhile, my more diversified portfolio only lost about 30% during that period. And over the next few years, our portfolios returns were similar (in the black). Meaning, he unwisely gambled on too concentrated a portfolio for passive investments and got burned.
You'd figure that since he was formally trained in the value of diversifying a passive investment portfolio, that he wouldn't have fallen into that trap. But, alas, his ego at the time convinced him that he was smarter-than-the-average-bear. And he ended up making a significant investing boo-boo.
https://vine.co/v/eut5nI3BFXL