Sabo's existence in One Piece is revealed during Marineford, where his amnesia prevented him from joining the battle. Oda shares a hypothetical scenario where Sabo saves Ace at Marineford ...

The Marineford arc of One Piece was, quite easily, one of the most important arcs of the whole story, and it does without saying that it remains, to this day, the most tense arc that Oda has dropped.