Twelfth Doctor (The Curse of Fatal Death)

A person who logically would have been called the Twelfth Doctor was the result of the regeneration of the Eleventh Doctor.

Biography
The Twelfth Doctor came into being when his previous incarnation regenerated after an electrical mishap.

He noted that he had wasted three incarnations in a very short time span simply because he "forgot to unplug". He proved the shortest-lived incarnation, only living long enough to shut off a zectronic energy generator before being hit by excess zectronic energy. He immediately died from the exposure as, allegedly, zectronic energy would kill Time Lords beyond any hope of regeneration.

He gave a final speech to Emma and the Master, asking them to "look after the universe" for him, as he had "put a lot of work into it". He promised to explain later, then died. His foes, the Daleks and the Master, vowed to reform and live better lives because of him.

Unexpectedly, he regenerated into a female incarnation. (NOTVALID: The Curse of Fatal Death)

Behind the scenes

 * Russell T Davies later offered the role of the Ninth Doctor to Hugh Grant, who declined, thinking the show would not be a success.
 * The line, "I've put a lot of work into it", would later be incorporated into the television story The Eleventh Hour, also written by Moffat, spoken by the true Eleventh Doctor, referring to Earth while responding to the Atraxi's comment that he was not a native of the planet. The full line was also stated by the mainstream Twelfth Doctor in the Series 9 prelude webcast Prologue.