Michael Corleone: Fredo, you're my older brother and I love you. But don't ever take sides against the Family again. Ever.
In the scene when Michael visits his father in the hospital in 1945 there's a NYC Fire Dept. Sign on the wall with the name of the Commissioner, Robert O. Lowery, who was commissioner from 1966 to 1973.
The baby used in Michael and Kay's baby christening scene is Sofia Coppola, Francis Ford Coppola's daughter. Sofia later had a prominent role in Part III as Michael's daughter, Mary.