4. Regeneration Disabledhttp://www.youtube.com/watch?v=EhMdOuN6h6o In the Christmas special, it was officially explained because of the War Doctor's existence and Ten using two regenerations, that made Matt Smith the 13th Doctor. And that meant he could no longer regenerate, staying true to previously established Time Lord rules of regeneration. This revelation has an interesting impact on the controversial "Let's Kill Hitler" episode, in which River Song poisons Eleven. Slowly dying, the Doctor stumbles back into his Tardis, where he begins a hilarious conversation with a young Amelia Pond voice interface. Once she confirms that he will in fact die in 32 minutes, the 11th Doctor shrugs it off and assumes everything can be fixed with a regeneration. The response? Regeneration disabled. At the time of this episode, some fans voiced concerns of lazy writing. But whether it was intentional or not, this now makes perfect sense. Eleven doesn't know it just yet because he hasn't met the War doctor, but he can no longer regenerate, and the Doctor almost didn't make it to Trenzalore.