Warren Buffett's Berkshire Hathaway (NYSE:BRK)(NYSE:BRK) disclosed a new stake in UnitedHealth Group (NYSE:UNH) in August, but after an initial rally, the stock has since fallen, putting the position underwater.Buffett's Berkshire Bets on UNH StockA 13F filing from Berkshire Hathaway unveiled the purchase of 5,039,564 UNH shares by the conglomerate in the second quarter, a position that represented one of the largest new buys from the company, then led by Buffett.The purchase of UnitedHealth shares in the s ...