The Fifth Elephant/Fridge


 * Fridge Brilliance: Vimes is bemused by the fact that the Low King is "crowned" by sitting on top of a sacred relic. But dwarfs regard things that are "low" to be better than things that are "high", so putting the Scone on his head would be treating it less respectfully than his perching on top of it.
 * Gavin and Carrot's fight with Wolfgang. Much is made of the fact that Carrot fights "by the rules"; but when Gavin jumps in, he's trying to fight fair, too. Which Gaspode notes before he jumps in to help.
 * Fridge Logic: In contrast to Pratchett's usual genius with consistency and detail, the weight mechanism guarding the Scone of Stone only makes sense if you don't think about it at all. If you try to figure out how exactly it's supposed to work, you'll find its apparent principle is convoluted enough to require a computer to run it, but it would still have to be a very stupidly programmed computer.
 * Which might have been the point all along, given that the overly complex mechanism proves almost idiotically easy to bypass.
 * The platform is - unseen to anyone walking on it - actually floating in an underground reservoir of water, so when you step on it (entering the Scone room), it displaces a certain amount of water, which flows into a bathtub-shaped tank on rails. When you step off the platform, that tank is shunted along a bit, and replaced by an identical but empty one. The original reservoir refills using a ballcock mechanism. The next time you step on the platform (on your way out of the Scone room), the same thing happens, displacing water into the second movable tank. If you were the same weight each time, the two tanks weigh the same now. Otherwise, one is heavier. The tanks slide sideways onto a simple scale mechanism. If there's a difference in the weights, the scale tilts, knocking a switch that lowers the cage around you and sounds the alarm-bell. Simple.
 * But it would be mechanically impossible for the components to move fast enough to get separate readings for Vimes, Cheery and Dee, and the system must also be able to distinguish between which of the people who went in is now coming out.
 * The story takes place on (in?) a flat world that flies through space on the back of four (formerly five?) elephants and a giant turtle. Besides, undoubtedly there are dwarven wizards.
 * Vimes notes that Igors tend to walk with a limp, Discworld wiki claims they do this intentionally, but given their tendency toward self improvement and how they're described as "good catches" there may well be an alternate explanation.