When the First Doctor, played by David Bradley, discovers the Twelfth Doctor, played by Peter Capaldi, he states that he thought he would regenerate into someone younger, at which point Capaldi insists "I AM younger!" While it is true that Capaldi is younger than Bradley, he is actually one year OLDER than William Hartnell was when he played the First Doctor in The Tenth Planet.
There is at least one scene where David Bradley speaks rather slowly, as if he is trying to remember his lines, which is very accurate to the state Hartnell was in when he shot The Tenth Planet.
Although The Tenth Planet: Episode 4 (1966) and The Power of the Daleks: Episode One (1966), the episodes that originally included the first regeneration, are both among the series' missing episodes, the clip of the regeneration itself survives because it was used in Episode #17.17 (1973).
David Bradley "returns" as the First Doctor. It will be the third time Bradley is part of Doctor Who. He played Solomon in Dinosaurs on a Spaceship (2012) and played William Hartnell in An Adventure in Space and Time (2013)
When the 12th Doctor retrieves brandy from his cabinet, the bottle towards the rear of the cabinet reads 'Aldebaren'. River Song retrieves a bottle of Aldebaren brandy from one of the TARDIS' roundels in The Husbands of River Song (2015).