Late in July 2012, Phil Mickelson, one of the world's most famous golfers, received a phone call from a well-known professional sports gambler, William "Billy" Walters. At the time, U.S. authorities say, Mickelson owed Walters a gambling debt, and Walters had a hot stock tip: buy shares in the food company Dean Foods Co. Four days later, Mickelson owned $2.4 million worth of Dean Foods shares, according to the U.S. Securities and Exchange Commission. A few weeks after that, he reaped a $931,000 profit when the company announced a spinoff that sent its share price soaring. Mickelson then paid off...