Seventh Doctor's regeneration

The Seventh Doctor underwent his regeneration into the Eighth Doctor in Walker General Hospital on 31 December 1999. Following a non-fatal shooting, the Doctor had been taken to the hospital where subsequent exploratory surgery by Grace Holloway with a camera accidentally clogged a vein; the anaesthetic he had been given delayed the regeneration by several hours almost destroyed the process entirely. (TV: Doctor Who) This was the seventh regeneration of the 12-regeneration life cycle beginning with the First Doctor. (TV: The Time of the Doctor)

Anticipation
After nearly drowning in the oceans of the Artifact, the Seventh Doctor briefly began to regenerate, but the process was averted when Mark Bannen applied artificial respiration. (PROSE: Parasite)

While trapped in his own mind during his confrontation with the Scourge, the Seventh Doctor and Benny Summerfield saw the Doctor's eighth incarnation, which the Doctor noted was more of a possibility than a reality when Benny became attracted to the image. (AUDIO: The Shadow of the Scourge)

Behind the scenes
Alixion would have ended with the Seventh Doctor regenerating due to psychic tension from facing the Manager, with Andrew Cartmel and Robin Mukherjee envisioning McCoy's Doctor regenerating "straightjacketed and gagged in a confinement cell". (DWM 255)