This raises a point which has always been a niggle at the back of my mind. If the Rosatos' failed attempt on Pentangeli's life was genuine, why would they even bother to give him the impression that Michael had ordered his death? Surely they could not themselves have been under any such illusion?
I believe Turnbull said it best when he pointed out that likely the line "Michael Corleone says hello" wasn't meant for Pentangeli but meant for the barkeep (Richie? I forget his name). That way later when questioned by the cops, he could say that the killer said "Michael Corleone says hello", and boom Michael now has all sorts of legal trouble.
