It was big news around the neighborhood that Gambino slapped Colombo across his face because he was pissed that Joe was bringing too much publicity to the Mob. Some time later I read an account of this story in a (wonderful) book by former Deputy Police Commissioner, Robert Daley, entitled "Target Blue".

Daley wrote that two weeks before the hit Colombo was sitting with a bunch of his guys in Monti's Bar in Brooklyn on either June 14th or June 15th when Carlo Gambino and his entourage entered. Gambino yelled at Colombo to "cut the crap" and at the end of the speech slapped Colombo hard on the face. Colombo had to just sit there and take it.

The book just backed up (with some details) what I had heard before so I'm confident that story is true. I don't mean to suggest that this is proof that Gambino was behind the hit, but it certainly makes it look like Gambino was pissed at Colombo.