Sixth Doctor's regeneration

Though the circumstances which led to the event were disputed, (PROSE: Spiral Scratch, AUDIO: The Brink of Death, Zagreus) the Sixth Doctor underwent his regeneration into the Seventh Doctor within the control room of his TARDIS shortly after the craft had been brought to the planet Lakertya by the First Rani. (TV: Time and the Rani) This was the sixth regeneration of the 12-regeneration life cycle beginning with the First Doctor. (TV: The Time of the Doctor)

Behind the Scenes
The Sixth Doctor's regeneration, depicted in TV: Time and the Rani, is perhaps the most infamous in the franchise's history.

Due to Colin Baker being fired from the role, when asked to film a single episode for the beginning of Season 24, Baker refused. He believed it would not be justified for him to be viewed as the current Doctor for a sliver of 1987 when he had been forcefully removed from the series.

So, when McCoy was cast as the Seventh Doctor, the choice was made to film a regeneration sequence with the actor playing both incarnations during the transformation. By coincidence, the stage production Doctor Who and the Daleks in Seven Keys to Doomsday had done a similar trick in 1974, having Trevor Martin play both the Third Doctor and a new Fourth Doctor via a white wig which was then hidden.

During the sequence, a white swirling mist was added to the actor's face in post production. Out of all the Classic Series regenerations, this effect is the most similar in style to the "Vortex energy" regenerations commonly seen in the BBC Wales run post-2005. As the white mist fades, so does the curly wig McCoy was wearing.

One notable discrepancy noted by both cast and fans is that McCoy is much smaller in stature than Baker, making it appear that the Sixth Doctor "shrunk" right before regenerating. This is actually included in the plot of PROSE: Spiral Scratch, as the Doctor is said to have diminished in height due to being weakened over the plot of the story.