Tenth Doctor's siphoned regeneration

"I'm sorry, it's too late. I'm regenerating."

- The Tenth Doctor's last word

During the Planetary Relocation Incident, the Tenth Doctor was fatally shot by a Dalek, triggering a regeneration which was siphoned into the Tenth Doctor's hand. (TV: Journey's End) By later accounts, this was the eleventh regeneration of the twelve regeneration life cycle beginning with the First Doctor. (TV: The Time of the Doctor)

Legacy
In the 1851 incident, the Tenth Doctor briefly believed that a man who believed himself to be the Doctor could indeed have been "the next Doctor. Or the next but one. A future Doctor anyway." It soon became apparent that the other Doctor was a human named Jackson Lake, who had been given the Doctor's memories by an infostamp. (TV: The Next Doctor) By the time of his retirement to London in 1892, the Eleventh Doctor believed himself to be "the last Doctor" and so was initially skeptical as to the Twelfth Doctor's identity when they met in an encounter which the earlier Doctor lost his memory of. (AUDIO: Regeneration Impossible) During the Siege of Trenzalore, the aging Eleventh Doctor revealed to Clara Oswald that he had expended all twelve of his regenerations. After reminding her that the War Doctor was still a regeneration despite not identifying as the Doctor, he added that "number ten once regenerated and kept the same face", which he attributed to "vanity issues". (TV: The Time of the Doctor)

In an alternate timeline, the Tenth Doctor, after meeting the Eleventh Doctor, was skeptical of the Twelfth Doctor, knowing that he should have only one regeneration left. The Eleventh Doctor was similarly skeptical of his supposed successor but eventually became content about him, voicing his relief to Clara Oswald that he would not be the last incarnation. (COMIC: Four Doctors)