Tenth Doctor's regeneration

The Tenth Doctor, who had previously regenerated and kept the same face during the Planetary Relocation Incident, (TV: Journey's End) met his regeneration into the Eleventh Doctor as a result of radiation poisoning intentionally incurred in order to save the life of Wilfred Mott after thwarting 's Final Sanction. (TV: The End of Time) By later accounts, this was the twelfth and final regeneration of the 12-regeneration life cycle beginning with the First Doctor. (TV: The Time of the Doctor)

Anticipation
After the Doctor and Donna Noble aided in the revolution of the Ood in 4126, Ood Sigma ominously warned the Doctor that "[he thinks his] song must end soon". (TV: Planet of the Ood)

After he brought The 200 London bus and its passengers back through the wormhole from San Helios, Carmen, a passenger with precognitive psychic abilities, echoed the words of Ood Sigma, telling the Doctor that his song was ending, adding: ""It is returning. It is returning through the dark, and then Doctor...oh but then... he will knock four times.""

- TV: Planet of the Dead

Shortly before his regeneration, the Tenth Doctor saw a solitary pale Watcher observing him; shortly after the TARDIS dematerialised, the Watcher also disappeared. (PROSE: The Advent of Fear)