So I found this on some forum. Supposedly the spoiler for 303 (it's in Japanese).
Tried running it through a translator and from what I can tell, Mayuri and Szayel trash talk each other, but something happens and everyone is surprised. Cut to Byakuya, who is being healed along with Hanatarou and Rukia. The old Espada suddenly appears, but is impaled through the chest when Gin comes up behind him and releases his sword. He smiles and the chapter ends.
Two questions if this is true:
1) Did another one of the top 3 Espada (or even Tousen or Aizen) show up at the Mayuri fight?
2) Why the hell did Gin kill the old Espada? Is he betraying Aizen? (not likely) Are the 3 traitor captains killing off all the Espada to make room for the Vasto Lorde Espada Aizen has been hiding this whole time?