From Barron's via CNBC:
Doug Kass, who famously shorted "everything related to housing" in 2007, told Barron's he has a short position in Warren Buffett's Berkshire Hathaway, largely because the company has suffered from a drift in its investment style.
Kass, founder and president of Seabreeze Partners Management, said that while previously calling derivatives "financial weapons of mass destruction," Buffett had out of character immersed himself in several large such transactions that led to an unrealized $1.6 billion loss in the company's first quarter.
He said he is also short Berkshire
[BRK 122400.0 --- UNCH (0%) ] because he believes the salad days for the insurance industry, a major part of Berkshire's business, are over, and the company's premium valuation may erode as some financial services companies regain their footing, Barron's reported in its issue dated May 19.