** Old Kazran and Kazran's dad are both played by Creator/MichaelGambon. Leads to a heartbreaking moment when young Kazran calls his old self "dad?"
** Likewise, Young Benjamin, and the unnamed younger Pettigrew in the present are both played by Bailey Pepper.
* TheCastShowoff: Music/KatherineJenkins, better known for being a GlamorousWartimeSinger, plays Abigail. Of course she ends up singing.
* DevelopmentGag: Eleven produces a picture of himself with Einstein towards the beginning of the special. Creator/MattSmith wrote fanfiction about the Doctor meeting Einstein after being cast to prepare himself for the role.
* GeniusBonus: It's recently been discovered that great white sharks really ''can'' live at least 70 years, to judge by growth-ring patterns in a large male salvaged after its death.
* YouLookFamiliar: The starship captain is played by Pooky Quesnel, who would later go on to play Dorothea Ames in the spinoff ''[[Series/Class2016 Class]]''.