** Old Kazran and Kazran's dad are both played by Michael Gambon. 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.
* HeyItsThatGuy: The Sardick father and son are both [[Film/HarryPotter Dumbledore!]]
** Looks like Hamish hopped aboard a time machine, got a job as a pilot and dyed his hair to finally get over [[Film/AliceinWonderland Alice's rejection.]]