Two things that never made sense...
I've watched this film countless times and there are two things that never made sense.
1. During the attempted assassination of Frankie Pentangeli, the assassin said, "Michael Corleone says hello."
Frankie survived and remembered hearing that and turns on Michael because of it. But why would the assassin say that? He intended to kill Pentangeli, after all. Why did it matter what a dead man heard? Was it just in case Frankie survived (as he did).
2. Michael tells Tom that he's going to be the Don. Makes a dramatic speech about why he had been left out of things and how he was the only one Michael could trust and Tom was going to be responsible for the safety of his family.
And then it's totally forgotten about. Never comes into play. We don't see Tom acting like he's the Don. It doesn't seem like anybody ever heard that Tom would be the Don. Tom is then seen acting as Michael's lawyer, not as the Don. Was this a subplot that got dropped?
Maybe this was put back into the "Saga", I don't remember.