Paulie called in "sick" the day Vito was shot, and the consensus was that he was in on the attempted hit. In the film, during the wedding scene we see that Paulie apparently works for Clemenza, but later I think Clemenza says something about having asked Fredo whether or not Paulie should be replaced, and Fredo said no. Clearly having Paulie around was a breakdown in security, and I wonder whether anyone got blamed for it. I know Clemenza was dispatched to shoot him, perhaps as a sign of loyalte. Then again when Tessio betrays the family, Hagen tells Mike he always thought the traitor would be Clemenza. Maybe that conclusion was based on the Paulie thing.

In the DVD deleted scenes, Sonny definitely suspects Clemenza of being involved in the assassination attempt on Vito, to the point where he only wants Tessio's men sent to the compound for security in the aftermath. I do not recall what he finds out to clear Clemenza of suspicion

He finds out, from his sources in the phone company, that it was Pualie who had made contact with those who shot Vito.

As far as who's to blame for Paulie, well, when I read the question this quote from Michael, when he is talking to Tom, came to mind :


"You see -- all our people are business men, their loyalty is based on that. Now, one thing that I learned from Pop was to try to think as people around you think. Now on that basis, anything's possible."


Don Cardi

Who is to blame? Paulie!

Good question, dt! Technically, Clemenza was liable, since Paulie was his guy. And as you noted, it was Clemenza's responsibility to whack Paulie (at least partly a loyalty test), and as Threlkis said, Sonny definitely could not dismiss Clemenza as complicit until he got the phone company records.
But, the bottom line was: the family was at war, and Clemenza was needed. Technically, hothead Sonny could have demanded that Clemenza fall on his sword (or stabbed him with the sword), but he needed Clemenza for the war.

Maybe it's too fine of a line, I think it's not exactly distrust of Clemenza that turned Santino to Tessio's men after the attempted hit, it's just simple prudent procedure. The lapse could have come from either sloppiness or disloyalty, but that was no time to find out.

I think that Hagan's remark that he would have expected Clemenza to be the traitor is out-of-character, and that it was made purely so that Michael could say something philosophical about how intelligence is a poor master.