In Part III, she thinks Fredo drowned. How can she believe that? Connie knew right away when Michael killed Carlo. Kay and Anthony obviously know Michael killed his brother ("Tony knows you killed Fredo"). Even Mary had heard rumors. The only possible explanation is that she didn't want Michael to know that she knew what really happened (if that makes sense).

(I noticed during the famed "Silent Scream", Al Pacino had a huge piece drool coming out the side of his mouth. Every time I see the ending now, that is always going to distract me. I just ruined one of my favorite movie endings.)