- Acting for Two:
- Almost every actor in the cast plays two characters — one character in the normal world, and their parallel world counterpart. The only major exceptions are Jon Pertwee himself, since there is no Doctor in the parallel world, and Christopher Benjamin, who plays Sir Keith Gold in the normal world, since his parallel world counterpart is already dead when the Doctor arrives there.
- Stuntman Derek Ware did not actually perform the stunt in which his character, the mutated RSF Trooper Wyatt, having been shot, falls to his death from the top of one of the chemical tanks, in case he was injured (since he was also needed for the subsequent studio recording). His place was taken by Roy Scammell, who (ironically) also played the soldier who fired the fatal shot. Ware stated in an interview that Scammell had already signed the contract to do the fall before Ware had been cast as Wyatt. At the time it was filmed, it was the highest fall ever performed by a British stuntman.
- Creator Couple: Director Douglas Camfield's wife Sheila Dunn plays Petra Williams.
- Creator's Favourite Episode: Both Nicholas Courtney and Caroline John named this as their favourite serial.
- Deleted Scene:
- A scene with the mirror universe characters listening to the radio was cut from the British broadcast, as the radio newscaster turned out to be too recognisable as Jon Pertwee doing one of his "silly" voices. The scene was included in the US broadcast, as it was thought that the audience there wouldn't be familiar with Pertwee's comedy work; this copy inadvertently made its way onto the VHS release of the story in 1994 (presumably because the only colour copy of the serial at the time was an NTSC version returned from Canada), but for the later DVD release it was featured as a separate deleted scene.
- Another cut scene would've actually shown the death of the UNIT soldier mentioned by the Brigadier: Harry was to have come up behind him and strangled him to death.
- Enforced Method Acting: The scenes on top of the chemical tank in Episode Two were not easy for Jon Pertwee, as he was afraid of heights. It took him fifteen minutes just to summon up the confidence to film the scene.
- Fake Australian: Greg Sutton was played by the English Derek Newark.
- Troubled Production:
- Director Douglas Camfield suffered a minor heart attack after completing the location filming and the studio scenes for Episodes One and Two. This left Barry Letts to direct the studio scenes for the remaining five episodes. Fortunately, Camfield's preparations were so meticulous, he merely followed Camfield's plans.
- During the chase scene on the parallel Earth, Bessie's bumper gashed open stuntman Alan Chuntz's leg as it passed as he jumped out of the way a fraction later than he was supposed to. The gash was so severe that he had to go to A&E. John Levene stated on the DVD Commentary that "it was the worst wound [he]'d ever seen" and after Jon Pertwee saw the wound and learned he caused it, he became sick and almost couldn't carry on filming until a few hours later. Pertwee even went as far as buying Chuntz's wife a gift to apologise for the accident.
- Wag the Director:
- Originally, Section Leader Shaw was to interrogate the Doctor in part three, but Nicholas Courtney, Douglas Camfield and Barry Letts thought that it should be the Brigade Leader. Caroline John wasn't happy about this.
- On another occasion, Jon Pertwee would not follow Camfield's directions and when the star argued with production assistant Chris D'Oyly John, an irate Camfield descended from the production gallery, forcing Sheila Dunn to intervene and persuade Pertwee to comply.
- What Could Have Been: Kate O'Mara was considered for Petra Williams. She would later appear in the series a decade and a half later as the Rani.
- Working Title: The Mo-Hole Project, Operation: Mole-Bore, The Mole-Bore and Project Inferno.
- You Look Familiar:
Trivia / Doctor Who S7 E4 "Inferno"