- Blooper: There's a creepy reveal when Omega takes off his mask and he has no body underneath it. Omega puts his mask back on, and flies into a Villainous Breakdown in which he's Chewing the Scenery so much that his mask flips up, revealing the actor's face, in closeup, just before the camera setup cuts to another angle.
- Fan Nickname: Omega's servants are often known as "Gel Guards" for fairly obvious reasons.
- Hostility on the Set: Terrance Dicks was once asked how Patrick Troughton and Jon Pertwee got on and he jokingly replied "Not at all". Initially, the two clashed over differing acting styles and working methods Troughton was fond of ad-libbing and goofing around, while Pertwee insisted on sticking to the script. After Pertwee questioned Troughton on one of his ad-libs, Troughton replied, "Instead of worrying about what I'm going to say, worry about what you're going to say". The two eventually did become good friends and would play up a mock rivalry at conventions.
- Milestone Celebration: The first multi-Doctor Adventure in honour of the tenth anniversary? ...yep, it's this trope in action! Though because it aired as the first story of the tenth season, and because the starting date of each season had shifted over the years, it actually aired only a month after the ninth anniversary.
- Reality Subtext:
- William Hartnell was unable to take on a role in the story due to illness, hence he's trapped in a time eddy and only being able to communicate with his successors through a viewscreen in the TARDIS. On top of that, it wasn't just his last appearance as the First Doctor, but his last acting appearance ever.
- The bickering between the Second and Third Doctors reflects the fact that Patrick Troughton and Jon Pertwee clashed over acting styles behind the scenes and initially didn't get on very well (they gradually warmed to each other over the years, however).
- Role Reprisal: William Hartnell and Patrick Troughton returned.
- Throw It In!: Nicholas Courtney's line about Cromer was improvised.
- Troubled Production: As well as William Hartnell's poor health, Patrick Troughton and Jon Pertwee did not get on well during recording, as Troughton's tendency to improvise whenever he thought that he could improve on the script made Pertwee uncomfortable. This had knock-on effects ten years later during the making of "The Five Doctors", with Terrance Dicks deliberately writing the script so that the Second and Third Doctors would not meet until everybody came together at the climax, which ended up disappointing both Troughton and Pertwee, who had become more friendly in the intervening years through doing fan conventions.
- What Could Have Been:
- In the original version, Deathworld, the Time Lords are in conflict with a Federation of Evil led by a personification of Death. To avert all-out war, the Time Lords manage to convince the Federation to allow them to send the three Doctors into the Federation's Underworld domain. There, the Doctors will do battle against various realisations of Death including zombies, the Four Horsemen of the Apocalypse, the Hindu goddess Kali, and the cyclops Polyphemus from Greek mythology with the victor in the contests determining whether the Time Lords or the Federation of Evil will prevail.
- Jamie was supposed to return and have a romance with Jo. Frazer Hines was busy with Emmerdale and his lines were given to Benton.
- Zoe was considered to return, but Jon Pertwee felt that too many returning characters would be distracting.
- The First Doctor was supposed to meet Two & Three in person and be personally involved with their adventure in the antimatter world and the confrontation with Omega. However, because William Hartnell's arteriosclerosis was taking a serious toll on his health when he was called up for the serial, the script was re-written to trap One in a time eddy, with his lines being split between Two & Three. A similar workaround would later be devised when Tom Baker rejected the chance to reprise the role of Four in "The Five Doctors".
- Working Title: The Black Hole.
- Written-In Infirmity: The First Doctor only appears on a video screen, recorded at Ealing Studios (not his home as legend has it), as William Hartnell was seriously ill and at times didn't remember he'd played the Doctor. He died two years later.
- You Look Familiar: The Chancellor of the Time Lords is played by Clyde Pollitt, who played one of the Time Lord judges who exiled the Doctor to Earth in "The War Games". This was deliberate on the part of the production staff, in order to create a link back to "The War Games".
Trivia / Doctor Who S10 E1 "The Three Doctors"