Factual error: When Vito is about to kill Fanucci during a street festival, it is a cold winter's day. Fires are burning in 55 gallon drums along the street and Vito is wearing a heavy overcoat under which he conceals his shotgun. But the scene begins with the shot of a banner that reads "Festa Di San Rocco" which is August 16, i.e. in the summer.
Factual error: When Michael meets with Hyman Roth at his home in Florida the television is on and you can hear the announcer calling a USC/Notre Dame game. The meeting takes place in the late 1950's. The announcer's voice belongs to Tom Kelly, a long time USC announcer, but he didn't start calling their games until the 1960's.
Factual error: When Michael meets Hyman Roth at his home in Florida, they sit in the room with the TV on. You can hear the football game being called by the commentator. The game is a USC/Notre Dame game with the teams called out by the commentator. The meeting takes place in the late '50's before Bautista's overthrow, but the game's commentator is Tom Kelly (very distinctive voice) who didn't begin calling USC games until the early '60's.