Trivia: The 2018 novelization, also written by Steven Moffat, includes a scene that he had wanted to include in the episode proper but couldn't due to rights issues, where UNIT head Kate Lethbridge-Stewart, while showing Clara around the Black Archive, shows her the posters of 1960s movies Dr. Who and the Daleks and Daleks Invasion Earth 2150 AD, which were loose adaptations of the first two Dalek serials, "The Daleks" and "The Dalek Invasion of Earth" starring Peter Cushing, and confirming them as existing in the Doctor Who universe. Kate then explains that the movies were made with the Doctor's knowledge, and that he was good friends with Cushing and loaned him a waistcoat to wear in the second movie. The novelization then goes on to add that the Doctor got in some trouble with UNIT for bringing Cushing to the future, to appear in a movie made long after his death - a nod to the late actor's controversial CGI resurrection in the Star Wars spinoff movie Rogue One.
Trivia: When Clara jumps into the Doctor's timeline and is shown encountering the Doctor - usually indirectly - in various incarnations and adventures from the classic series, the adventures chosen were mostly ones regarded as being badly written and/or having nonsensical plot elements, with the implication that in-universe, those plot elements were due to the Great Intelligence's meddling. Examples include the notorious instance of the Seventh Doctor dangling himself from a cliff for no reason from the serial "Dragonfire."
Trivia: The Great Intelligence, the episode's villain, first appeared in a pair of Second Doctor serials, "The Abominable Snowmen" and "The Web of Fear". As a result, there are a few references to the two stories in the episode. First, the Doctor places pieces of the ice governess in a metal lunchbox from the '60s with a map of the London Underground on the lid. He then shows the box to the Intelligence while it is still in the snowglobe, prompting it to ask what the markings mean. The Doctor tells it, and then remarks that the Underground is a "weakness in metropolitan living" because he dislikes tunnels. In "The Web of Fear", the Intelligence and its minions set up in the Underground in the late '60s, with this episode's implication being that, as those events have yet to occur for the Intelligence, the Doctor unwittingly gave it the idea. Second, at the end, the Doctor takes another look at Dr. Simeon's business card for the Great Intelligence Institute and finds the name familiar, but cannot place it, as the Great Intelligence was not actually addressed as such in the episode. When this episode was made and aired, both "The Abominable Snowmen" and "The Web of Fear" were completely lost, with no copies in the BBC archives - a situation that changed less than a year later.
Trivia: This episode was originally titled "The Last Cyberman." The title was changed to something less spoilery after one of the guest stars forgot their copy of the script in a London cab. The script was swiftly returned, but the showrunners decided to change the title anyway out of caution.
Trivia: The events of the previous episode, "A Town Called Mercy", actually take place during the events of this episode, specifically at the end of the seven-week anniversary trip the Doctor takes Amy and Rory on. The evidence? In "A Town Called Mercy", the Doctor mentions Rory having left his cell phone charger in King Henry VIII's ensuite. During the montage showing the anniversary trip in this episode, the Doctor, Amy and Rory are shown hiding under a bed in the past, and a whispered conversation reveals they are hiding because Amy just accidentally married King Henry VIII.