This is the first time I noticed this, but during the Connie's wedding, when Barzini arrives, a young Carmine Lupartazzi is right behind him smoking a cigar. Also at Connie's wedding, when Sonny is sitting at the table and picks up the cigarette, he drops ash on his tux. And when Michael murders McClusky, he shoots him the neck first, and the camera pans out, yet you can clearly see the bullet hole to his head, with a little trickle of blood in this shot, prior to the camera cutting back to the close up, the bullet hole not being there now, Michael shoots him again and it comes back.