I think Fredo knew in Vegas that the Don was semiretired. I think he blurted out his statement as a way of placating Moe Green--as if to say, "Gee, I'm sorry you and my brother are at an impass. Maybe my Dad can resolve it." Either way, it made Fredo look stupid--a sure sign of disrespect.
Another: In the flashback scene at the end of GFII, after Sonny complains about the Japanese bombing Pearl Harbor on Vito's birthday, Fredo replies, "The Japs didn't know it was Pop's birthday." Duhhh... He also shakes Michael's hand after Michael says he joined the Marines--and Sonny slaps his hand away.