World and lore
Check that a treasure map's clues point to exactly one spot
Four steps from the mill and four from the oak sounds precise, until the player finds three spots that fit and digs up the wrong one. Here is a check that lists every spot matching the clues, so you know whether a riddle has one answer, several, or none.
Check that a villager's day does not put them in two places
The baker opens the shop at noon and is at the market at noon, or walks from the mill to the church in no time at all. Here is a check for daily routines that finds overlaps and impossible walks, including the night shift that runs past midnight.
Tell where a wandering trader is on any day so the player can find them
The trader who sells the one rare tool is never where the player looks, because nothing says where he is on which day. Here is how to give him a fixed round of stops and roads, answer where he is on any day, and tell the player the next day he will be in a given town.
Make weather change in believable steps and stay long enough to notice
Left to chance, the sky jumps from a clear day to a storm and back within an hour, and snow falls in midsummer. Here is a filter that takes any stream of random weather picks and turns it into weather with sensible neighbours, a minimum stay for each kind, and seasons that ban some kinds.
Find the next market day when holidays can move or cancel it
The market is held every seventh day, except that on a holy day it moves to the day after, or does not take place at all. Here is how to answer "when is the next market" correctly, including the awkward case where the moved market lands after the day you asked from.
Work out which shops and quests vanish when a town is destroyed
Burn down the harbour and the fish seller should go, and the delivery job for him, and the boat trip to the island, but not the other road to the island. Here is a tidy way to list everything that stops, in any order of events and without leaving loose ends.
Decide who inherits when the ruler dies, exiles and children included
The king's eldest son is dead, his own daughter is nine, and a younger brother is in exile. Who rules? Players ask and the AI invents an answer each time. Here is a succession rule that gives the same answer every time, with a regent when the heir is too young.
Work out which animals die out after you remove one from the wild
A plague takes the hares, and the story says the foxes thrive on mice anyway, while the owls are left with nothing. Here is how to write down who eats whom and follow the chain, so that the world's reaction to a loss is right, including animals that eat each other in circles.
Catch the world up after days away so it equals playing every day
The player comes back after a long sleep and the farm has either not changed at all or gained so much that raids and storms seem never to have happened. Here is how to replay the missed days in order so the result is the same as if the player had never left.
Keep a town's history as a log of events so it cannot become impossible
The town burned down in one chapter, is rebuilt in another, and in a third it burns for the first time again, or is rebuilt before it was ever destroyed. Here is a way to store only the events and derive the present state, rejecting ones that cannot follow.
Save a generated world as small changes and survive a patch
Saving every tile of a huge world makes the save file enormous, and saving only the player's edits breaks the moment you improve the world generator. Here is a way to store just the changes, drop the ones that undo themselves, and spot edits a patch has made stale.