If you check out this "What If" cover for chapter 596 you can easily notice Ace's tattoo is different from the one he always had throughout the manga . Every time we saw besides this one it was spelled like this ASCE with the S crossed out. In the chapter where Sabo wrote a letter to Ace an Luffy telling them he was leaving you can see the S with the cross through it is Sabo's pirate mark. Also it is seen on his flag as he tries to leave out for sea. This denotes that Ace knew about what this mark meant and had it tattooed on him as a sort of way of paying respect to his supposedly fallen brother and friend. When Sabo's mark was shown to be a crossed out S most people came to the conclusion that's why the crossed out S was in Ace's tattoo. Although it was a widely spread rumor that Oda had supposedly told in an SBS that the crossed out S was a mistake by the tattoo artist. This has not been confirmed and therefore it makes perfect sense that it was in honor of Sabo the whole time. Now with the realization that if Sabo was alive and grew up with Ace and Luffy then Ace's tattoo would be different and it is in this cover page. I think this is Oda's way of telling us he knew about Sabo the whole time since the introduction of Ace and goes to show us how great a writer he is. This also must mean Oda plans out his story very far in advance and knows where it will be going as an overall story. Or of course he could just be retconning it to fit what he has just not written. Maybe we will never know, but to me it just reassures me of Oda's great story telling ability. This is why his story continues to amaze and delight us all , because he actually knows where he is going with it. Let me know what you think in the comments and such and feel free to pass this blog post around to anyone you feel who would be interested in such things.