I just came up with this, because I am trying to understand how in the hell did Madara survive that (fatal as it seems) hit. Hashirama is no noob, when he strikes like that with the intention to kill, he hits a vital spot.
So Hashirama couldn't have missed.
We know that Izanagi is out of the question because Madara managed to evolve two Rinnegan, thus he couldn't have sacrificed one to save his ass. The technique doesn't even work like that.
So Hashirama leaves and Madara remains there dead. By the way what kind of friend does that??
Someone said he tricked Hashirama with genjutsu. But what about the wound? He did get the wound and the wound is by a direct hit to a vital spot. So that doesn't work either.
So it seems we are just missing info here. Remember when Madara told Kabuto about the "secret" of his body? Was he really just referring to his Hashi boob? Or there is more?
Just throwing an idea, don't kill me :D