As a One Piece fan who doesn't read the manga I must ask this:
In this flashback when Ace meets Shanks, Ace says:
"My brother was always talking about you as his lifesaver"
As you are aware, Shanks lost his arm when he saved Luffy. However, at 0:43 you can clearly see both of his arms.
Does this happen in the manga or is it just an animation mistake?