Trivia: Kate's comment, "Five rounds, rapid," refers back to a command that her father Alistair gives when firing at a stone gargoyle in the original series serial "The Daemons."
Trivia: A portrait of the First Doctor is on the wall of the UNIT safehouse.
Trivia: The Naval surgeon that Kate refers to as having dealt with in a previous Zygon invasion would be Harry Sullivan, a companion to the Fourth Doctor.
Trivia: Kate being unable to place the previous Zygon invasion to a specific decade refers to the confusion in timing of the UNIT stories. Some episodes claimed the stories took place in the 70s, while others placed them a decade later.
The Doctor mentions the Terileptils starting the Great Fire of London. This refers back to the original series story "The Visitation," where it is revealed that the aliens (with a slight assist from the Doctor) started the Fire.
Trivia: The Doctor's comment about reversing the polarity of the neutron flow, and "I bet that means something, sounds great," refers to his third incarnation's habit of using it as an explanation for his technical adjustments.
As this episode opens we see bi-planes with laser guns, and soldier with gas masks along with bows and arrows. This was first seen in "Genesis of the Daleks, Part One", as the Doctor, Sarah, and Harry wander through the wasteland and realize the war has been going on for quite some time, musing that they probably started out with all modern equipment but ran out of resources and had to make do.
Trivia: The TARDIS and its keys being vulnerable to lava dates back to the original series episode "The Mind Robber", when a volcanic eruption threatens to destroy the TARDIS.
Trivia: Clara's birthday is November 23, the same day that Doctor Who premiered in 1963.
The train is a Class 117, which was decommissioned in 2000. (The episode was filmed on a heritage railway, accounting for the discrepancy.)
Trivia: The Doctor offers Moorhouse jelly babies. The Second Doctor ate them on several occasions and they first were seen in "The Dominators". However, they are commonly associated with the Fourth Doctor, who offered them to friends and enemies alike.
Trivia: The Doctor's use of the phrase "Are you my mummy?" refers back to the first series episode "The Empty Child," when the transformed Child used the same phrase.
Trivia: The Doctor's comment about there being so many artron emissions near Coal Hill refers back to the original series episodes "An Unearthly Child", "Attack of the Cybermen", and "Remembrance of the Daleks".
Trivia: Among the images that Psi brings up of thieves and villains are: a Sensorite, the Gunslinger, a Tereleptil, Abslom Daak from the Doctor Who comic strips, an Ice Warrior, Captain John Hart (from Torchwood), Androvax the Trickster (from The Sarah Jane Chronicles), and a Slitheen.
Trivia: The Doctor's line about Sontarans perverting the course of human history is from the 4th Doctor's first spoken words when he wakes up from his regeneration in "Robot". The 4th Doctor in turn is referring back to the events of "The Time Warrior" and his first onscreen encounter with the Sontarans.
Just after Clara leaves the TARDIS, you can see that the door is closed. During the fight, the door opens and closes.
Trivia: The Doctor suggesting that they might be in a Miniscope refers to the original series episode "Carnival of Monsters". The Miniscope is a device that gathers people and objects from a narrow period of time and stores them, allowing viewers to watch the sequence of events and repeats them as they wish.
Trivia: The clockwork robots using spare parts to rebuild a spaceship, which the Doctor can't quite remember, is from "The Girl in the Fireplace" in series 2 of the new series.
Trivia: A dinosaur walking down the Thames, albeit with much worse special effects, was last seen in "Terror of the Zygons" episode 4.
Trivia: The Doctor's reference to a scarf goes back to his fourth incarnation, who wore that piece of apparel most of the time no matter what circumstances.