These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: The Last Continent
Fridge Brilliance: There's a passing mention in Hogfather that pencils in Discworld aren't manufactured, but grown as stems of a type of bush native to Sumtri. Considering how many other utensils started sprouting from the Mono Island vegetation, it's quite possible that this is where that bush's ancestors came from.
More Fridge Brilliance: Ponder's and Ridcully's discussion of how stepping on an ant might change the future takes on new depths of irony when you realize that they're only having the conversation because of a single ant. Specifically, the one that the Lady arranged to divert from its course through Hex's tubing, at the end of Interesting Times, thus causing Rincewind to be teleported to Fourecks rather than back to UU. If not for that one ant, the faculty wouldn't have had any reason to visit the office where the portal to Mono Island could be found, in the first place.
Fridge Brilliance meets Continuity Nod: When fleeing from the police in Bugarup, Rincewind is dismayed and amazed by how the pursuing officers stay on his tail. Aside from a very brief sojourn in Interesting Times, Rincewind hasn't been in Ankh-Morpork since the events of Sourcery, so has completely missed the rebirth of his home city's Watch in the wake of Guards Guards!. He only remembers how lazy and fearful the Watch used to be, and is probably quite astonished to see what's become of them once he gets home at last.
Fridge Logic: The god of evolution mentions having had believers at some point, as well as experimenting on a cow he wanted to make more inflammable. If so, how does he not recognise the wizards as humans or, indeed, not know the most popular way of producing new generations, or even about different sexes? What sort of believers (or cows) did he have?