It's so bad that NYC waiters know to never seat these two next to each other at a restaurant.
Ackman and Icahn feuded back in 2004 over a quick deal they made when Ackman's firm was being investigated by the SEC in 2003. Ackman cold called Icahn and asked him to buy shares of Hallwood Realty, a real estate company trading for about $60, but Ackman said was worth $140. Ichan agreed, but when Halwood merged with another company for $137 a share, Icahn wouldn't give Ackman a cut.
So, as is common on Wall Street, there was a lawsuit. As is less common, Ackman called "shakedown artist" whose word was "useless" and also convinced another investor to refuse Icahn's money.
Ackman won ultimately, and Icahn paid him $9 million.