Harpo Does Something Funny: In the script for "The Hall of Dolls", the infamous "Eeny Meeny Miney Mo" scene simply dictates that the King of Hearts closes his eyes and recites the rhyme, without giving any specifics. Campbell Singer was the one who chose to use the n-word version, which was common in the UK at the time since Rudyard Kipling had popularised it in 1935.
Missing Episode: All except the fourth, which was recovered by ABC in Australia in 1986. It survived because its title, "The Final Test", had led it to be mis-shelved among the station's cricket footage.
No Budget: Because "The Ark" had gone over-budget, this one was made on the cheap.
Prop Recycling: The Toymaker's robes (a stock outfit from the BBC's wardrobes) were previously worn by the titular explorer in "Marco Polo".
Most of the problems arose because of the show’s staff having a shake-up. This resulted in a clash between then-showrunner John Wiles and script editor Donald Tosh, and the new team of Innes Lloyd and Gerry Davis who had to finish the story.
The story had to be rewritten from the ground up three separate times. One of them was to get rid of copyrighted characters that the author had changed his mind about using. The outgoing team of Wiles and Tosh wanted a more mature story focusing on the conflict between the Doctor and Toymaker. Lloyd and Davis however, wanted the story to be "much lighter [and] more pantomime", with a focus on the games Steven and Dodo went through. This clash continued even after Wiles had been let go from Doctor Who, including sending a memo to the BBC’s head of serials. This was worsened by the episodes’ lack of budget, and left the story with little structure. The one good thing about this time was that Lloyd and Davis had a better relationship with William Hartnell, whose health problems had made him more feud with cast and crew.
Even after the episodes were completed there were difficulties. After airing "The Dancing Floor", The BBC had to deal with charges from the estate of Charles Hamilton that the character of Cyril was supposed to be Billy Bunter, and and an evil version as well. This was worsened because the actor who had played Cyril, Peter Stephens, had ad-libbed the line "My friends call me Billy" during recording. The BBC had to make a Continuity Announcement at the end of "The Final Test" stating that Cyril was only based on Billy Bunter.
This came very close to being the show's first regeneration story. Producer John Wiles was having major problems getting along with William Hartnell, and decided to get rid of him via the Doctor being turned invisible for most of the story — the idea being that when he appeared again, he would be played by a new actor. However, the BBC didn't approve of this plan in the least, seeing Hartnell as integral to the show, and when Wiles refused to back down they responded by firing him. Which in turn resulted in another case of this trope...
The original storyline for the serial was very different, and much more adult and satirical in scope. After he was fired, however, Wiles massively overspent on the previous story as a final act of spite against his superiors, leaving the incoming team of producer Innes Lloyd and script editor Gerry Davis virtually nothing to make this story with. Davis therefore had to do a massive rewrite, cutting the story right down in order to accommodate filming with as many pre-existing sets and costumes as possible, changing the story's tone from satirical to just flat-out bizarre.
The original script featured the main characters from the play George And Margaret. The gimmick of the story was that, despite revolving around their arrival, the play ended just as they were about to appear. The idea was to have them finally appear, only to torment Steven and Dodo. When the author Gerald Savory denied the characters’ use (just as they had been cast), their roles were filled by others.
According to Donald Tosh, the commissioning script editor and (uncredited) co-author of "The Celestial Toymaker", the intention was that the Toymaker was, like the Monk who had predated him, a member of the Doctor's own race.
Word of God: Donald Tosh confirmed in an interview that the Toymaker was merely dressed as a mandarin to fit in with him playing Tower of Hanoi. This is supported by the fact surviving materials make clear that the Toymaker’s minions are all specifically dressed for the games they take part in.
