DC's observation that Fredo held some limited sway over Clemenza is a good one, and it is interesting because Paulie's absence was the key to the Don being shot. If Fredo controlled Clemenza's choice of bodyguards (and did so in spite of Clemenza's clear doubts about Paulie ---"What are you a dance judge? Go do your job.") it could have been a sign to outside observers that "the Don .... was slippin'...." and that Sollozzo couldn't have gotten to him ten years ago.