Weather that is never saved
It started as a much smaller request: a few bugs drifting over the catching areas so the overworld felt inhabited. Building that as a one-off would have been quick. Building it as a general particle engine meant the same code could also carry mosquitoes over the lakes, leaves coming down in autumn forests, a slow mist in the caves, and, once it existed, the sky itself.
The game already had a word called weather, and it meant something else: a fixed reading of how cold, warm or wet each patch of ground is. That never changes, and the stamina system leans on it. So the new thing is called sky weather everywhere in the code, and it adds to the ground's own reading rather than replacing it. Walking through a cold marsh in the snow is harder than either one alone.
Nothing about it is saved. Each day's sky is rolled from the world's seed and the date, the same trick that lets terrain regenerate byte for byte instead of being stored tile by tile. Load a save on a rainy afternoon and it is still raining, because it could never have been anything else. A day's weather also no longer runs midnight to midnight: it opens at a rolled hour and lasts a rolled few, so a clear morning can turn wet by lunch.
Two things only showed up after it shipped. New games kept opening in rain, often enough to read as broken even though the odds were fair, so the first day of every world is now simply clear. And writing the last missing test file found a real bug: switching from one kind of weather to another cleared the old particles but kept a spawn credit that had been quietly piling up the whole time the field sat full, so the new weather arrived as one sudden burst instead of fading in.