Sam Giancana
At one point, Chicago mob boss Sam Giancana and his men wanted to kill Sinatra. Apparently, they tasked Sinatra with getting JFK to stop the crackdown on organized crime. An FBI member had wiretapped a member of the mob and recorded him saying, "Let's hit Sinatra. Or I could whack out a couple of those other guys." The two ended up with a mutually beneficial deal.
Howard Hughes
In 1967, billionaire Howard Hughes bought real estate and ended up cutting off multiple performers from their income. This also ended up cutting off Sinatra's credit line at the Sands. This caused Sinatra to move to Cesar's Palace, but Dean Martin and Sammy Davis Jr. remained at the Sands.