This is a great article from WSJ, below are two excerpts – full piece is here

Dan Loeb , chief executive of activist-investing firm Third Point:

Worst: It happened while I was in college and trading stocks in my spare time. I had made $120,000 and proceeded to invest all of it in an idea I was particularly passionate about: Puritan-Bennett, a medical-respirator manufacturer. The company suffered massive losses after its products were associated with several deaths. In the years it took to remake my lost profits, I had a lot of time to absorb the important lesson of not overconcentrating positions.

Jonathan Tisch, chairman of Loews Hotels & Resorts:

Worst: In early 2000, I invested in the Chingalle restaurant in New York City’s Meatpacking District. Unfortunately due to a shaky period on Wall Street and then the impact of 9/11 on lower Manhattan business, the restaurant closed. Every time my wife and I eat in what is today a booming Meatpacking area, I lament how we were a little ahead of our time.

Article here