History Trivia / DoctorWhoS6E3TheInvasion

** Despite the later UNIT Dating Controversy, this story is firmly stuck in 1969.

* ArtEvolution: The TARDIS gains a real, opening telephone box with a handle on the left-hand door for the first time. Between the preceding story "[[Recap/DoctorWhoS6E2TheMindRobber The Mind Robber]]" and this serial, the prop had been borrowed by ''Series/OutOfTheUnknown'', which had installed it for a scene where a character called the police.

* CastTheRunnerUp: Creator/JohnLevene was originally hired to play a Cyberman, then the actor originally cast as Benton was sacked for tardiness.
* CreatorCouple: Director Creator/DouglasCamfield's wife Sheila Dunn was both the International Electromatics computer voice and the telephone operator.

** Rutlidge was originally supposed to be forced into shooting himself by Vaughn. It was filmed, but cut before broadcast, presumably for being too dark, and it is now lost.
* TheOtherMarty: Another actor was cast as Corporal Benton, but he was constantly late and delayed shooting. The director, Creator/DouglasCamfield, sacked him and gave the part to Creator/JohnLevene, an untrained extra and monster actor who he noticed got on very well on-set with Creator/PatrickTroughton and Creator/FrazerHines (having danced about in his Yeti costume for their amusement while shooting "[[Recap/DoctorWhoS5E5TheWebOfFear The Web of Fear]]" last season). The UNIT extra played by Levene (with a different costume to Benton) is still visible, crammed right at the back of shots behind more major guest characters. Levene had signed up to play an extra, and ended up being a companion to three Doctors.
* RecycledSet: Vaughn has a factory/headquarters in London, and a factory in the countryside which looks identical on the inside. Jamie points this out, and Vaughn says something like that efficiency and uniformity are important.

** Despite the UNIT Dating Controversy, it's ironic that the story pretty firmly dates itself to 1969 at the latest, since steam engines can be seen in some shots. Steam power was withdrawn on British rail in late '69.
* ThrowItIn: Creator/FrazerHines takes credit for the "KilroyWasHere" graffiti. The director objected until Hines pointed out that the building was built by British workmen and they have fun on the job with things like that.

** Cybermats were to have appeared in this story.
** In addition to St Paul's Cathedral, Creator/DouglasCamfield wanted to film the Cybermen marching outside Tower Bridge, the Houses of Parliament and Hyde Park.
** An action scene where UNIT recaptures Professor Watkins had to be scrapped because there wasn't enough time to film it. A scene in which Gregory is killed by the Cybermen in the sewers also had to be inserted into the same episode, because the character was originally shot by Benton during the battle.
** The story was originally conceived as a four-parter, only to have its length doubled when another proposed four-parter, "The Dreamspinners", fell through due to budgetary issues.

* Episode 4 ended up airing on November 23rd, unofficially making this serial the fifth anniversary of the show. Considering that this was pretty much a complete accident the fact that it ended up being the first appearance of {{UNIT}}, an institution who would become a major presence in the following years and be involved in every televised anniversary since, makes it oddly fitting.

* YouLookFamiliar: Kevin Stoney had previously played BigBad [[Recap/DoctorWhoS3E4TheDaleksMasterPlan Mavic Chen]].

* ArtEvolution: The TARDIS gains a real, opening telephone box with a handle on the left-hand door for the first time. Between "The Mind Robber" and this serial, the prop had been borrowed by ''Series/OutOfTheUnknown'', which had installed it for a scene where a character called the police.

* CreatorCouple: Director Douglas Camfield's wife Sheila Dunn was both the International Electromatics computer voice and the telephone operator.

* TheOtherMarty: Another actor was cast as Corporal Benton, but he was constantly late and delayed shooting. The director, Douglas Camfield, sacked him and gave the part to Creator/JohnLevene, an untrained extra and monster actor who he noticed got on very well on-set with Creator/PatrickTroughton and Creator/FrazerHines (having danced about in his Yeti costume for their amusement while shooting "[[Recap/DoctorWhoS5E5TheWebOfFear The Web of Fear]]" last season). The UNIT extra played by Levene (with a different costume to Benton) is still visible, crammed right at the back of shots behind more major guest characters. Levene had signed up to play an extra, and ended up being a companion to three Doctors.

** In addition to St Paul's Cathedral, Douglas Camfield wanted to film the Cybermen marching outside Tower Bridge, the Houses of Parliament and Hyde Park.

** This story was originally going to feature Professor Travers and Anne from [[Recap/DoctorWhoS5E5TheWebOfFear "The Web of Fear"]], heightening this story's status as a sequel to that one. When Mervyn Haisman and Henry Lincoln refused to give permission to use the characters, they were replaced with {{suspiciously similar substitute}}s in the form of Professor Watkins and Isobel.