Everything in the entire book ties together and leads logically to the conclusion. Every. Single. Thing. Even things you thought were throwaway one-liner jokes. Even lines you'd completely forgotten.
The Brandenburg Concertos didn't need to be dropped into J.S.Bach's mind. The story of the Brandenburg Concertos is an odd one: they are believed to have been written as a job application (to be court composer) by J.S. Bach in the early 18th century, sent to the court of Brandenburg and pretty much ignored—shoved in a cupboard and never played, and they were pretty much totally forgotten about for 200 years. They were found in the cupboard in 1925, after J.S. Bach's fame was assured. Musical historians in the real world have no doubt that J.S. Bach wrote them.
Time travel in this book simultaneously creates timeline alterations (Kubla Khan, Bach's music, etc.) and predestined time loops (the sofa, the albatross, etc.). At first, this might seem like a contradiction or even a Plot Hole, until you remember that Reg did tell them that there would be paradoxes.
A realization that qualifies both as Fridge Brilliance and Fridge Horror is that, in the process of getting Coleridge to forget the second half of Kubla Khan, Dirk accidentally gave him the idea for killing the albatross in The Rime of the Ancient Mariner. The horrific part is that this is probably what gave Wenton-Weakes the idea to kill Albert Ross in the first place. Just like Reg's success in saving the coelacanth inadvertently killed the dodo, Dirk's success in saving the human race might have inadvertently resulted in Ross being killed, at least in the original timeline.
In Season 2, the One-Eyed, One-Horned Flying Purple People Eater is a scary purple monster that attacks people. In the actual song, however, the monster is of unknown color, only eats "purple people," and is friendly to humans. The boy who created the Purple People Eater was apparently too young to understand the wordplay of the song and get the joke.
How did the Mage go from the Harmless Villain that Francis meant for him to be to the unstoppable menace we actually see in the show? Because Francis made a mistake when creating Wendimoor that he was then unable to correct - somehow, the Mage ended up with an entire army of Kellum Knights. He was supposed to have ten. No wonder all those battles he was meant to lose turned out to be easy wins for him.
Everyone in Blackwing is astonished that Bart did not kill Ken when she met her. When they first meet, Bart explains that she is led by the universe to people she is supposed to kill (with one notable exception.) It is also shown that if Bart encounters such a person, they die. Now, in the series finale Bart tells Ken that the universe is "sorta making her feel that she should kill him now" and Ken goes "But you won't, will you?" At first it seems like Ken has the upper hand because he is manipulating Bart's child-like nature. However, given that Bart is led towards people she's meant to kill and she was led to Ken does mean that she is meant to kill him: it's just a question of when she's supposed to do it. So his survival in the series isn't because Bart made a conscious decision and went against the universe, but it just not being his time to die when they first met.
Suzie Borten is a limping, pathetic doormat of a woman, yet she wears sweatshirts with oddly triumphant expressions on them, like, "If you go blind, it's because you're looking at a star." This creates some humorous juxtaposition. However, after several episodes, it's revealed that she used to be an Alpha Bitch, so all of those vainglorious declarations were originally made with a straight face.
If one were to look closely at the sigil in the intro; first glance, it appears to be a single drawing, yet it is actually made up of individual pieces, feeding in to the illusion. Everything is connected, even if it may not look like it. The individual pieces are all reflecting each other, meaning that they are the same. Just like Dirk and Bart.
When Dirk hears Mr Priest coming after him in Season 2, his terrified response is to crawl onto a bed and lie still. He later mentions that he's met Priest before but doesn't clarify the circumstances. Given that Dirk's first instinct upon seeing him again is to essentially put himself to bed and be quiet, that could mean Priest had been set on him back at Blackwing for non-compliance.
