In 2013, Icahn and Ackman engaged in a very public battle over the nutritional supplements company Herbalife. Ackman was convinced that the company was a pyramid scheme, while Icahn saw it as a legitimate business.
The two men traded barbs for months, with Ackman even going so far as to call Icahn a "bully" during one particularly heated exchange.
In the end, Ackman was forced to abandon his short position in Herbalife at a huge loss, while Icahn made a tidy profit.
The whole saga was a fascinating glimpse into the minds of two of the most successful investors of our generation.