Far to the south of the usual adventuring area, but noted in the game-world history and on its maps since day 1, is a large city called Tanquair. For ten or more years of play all I-as-DM needed to do with Tanquair was think about it in my own mind and occasionally remind myself it was still down there; the PCs never got within a thousand miles of the place and never really had any effect on it. (and even though its local language was always on my random-languages table nobody ever rolled it, which meant I didn't even have to worry about telling anyone about the place the language came from!)
Then a mission (actually the start of a covid-shattered adventure series) took them down to that part of the world; meaning I not only had to think more carefully about what made Tanquair tick, I also had to design what lay in and across the ocean to its south which up until then was "off the map" territory. A few months later Tanquair ended up becoming their away-from-home home base, forcing me to go into way more detail than I ever really expected to (I had wrongly guessed they'd use their long-range travel capabilities to continue operating out of their usual base, far to the north).
Fortunately for me, I'd always envisioned Tanquair as a fairly stable sort of place anyway, so updating it was dirt simple. More challenging (and interesting!) was designing the southern ocean and what lay around/across it: a bunch of small realms to the east and to the south a great big empire very loosely based on an amalgam of various proto-Chinese dynasties. Again, at the moment there's a near-zero chance of the PCs ever going there, though they've had all kinds of interaction with a few of its more-piratically-inclined citizens, but now they've heard of it I feel I need to keep it - and all the other realms surrounding that ocean - vaguely updated. (further, I'm now looking long and hard at that area should I ever want to run an entirely different campaign and recycle the same setting)
And even then they caught me off guard: I'd done a high-level map of that southern ocean* and because the PCs needed to triangulate a directional pull to a site in mid-ocean they on a whim picked a random coastal city in a small irrelevant realm to the east and said "we're going there to try our directional spell again". They had magical means of long-range travel that allowed this, and suddenly I'm trying to DM them in a new and strange city that until that moment had been nothing but a nameless dot on a map!
By the end of that session I knew a lot more about the kingdom of Bonbai and the city of Baique than I ever expected to.
Odds of their ever going back there again: near zero.
* - it's map 7 on this page:
Decast maps