8th doctor regenerates into Hurt.
Hurt is the most unstable doctor yet. He leads the timelords into battle. He wrecklessly uses "The moment" and ends the time war killing trillions.
The moment has also brought back the 8th doctor. He clearly remembers time as the Hurt doctor but at the same time the hurt doctor has been erased.
One of Moffat's story trick of "It happened and didn't happen at the same time"
8th somehow regenerates again and we get Eccelston.
this seems to be the most likely theory. and the best one i've heard.