World and lore
Find doors whose key sits behind the door itself
In an adventure with many locked gates, one wrong placement can make the ending unreachable, and you only notice after twenty hours of play. Here is a way to walk the whole map the way a player would and list every gate that can never open.
Let the lake freeze slowly and melt quickly, so crossing needs a cold spell
One cold night should not carry a wagon across the lake, and one warm afternoon should not undo a week of frost. Here is how to grow and lose ice from daily temperatures, with a different pace each way, and to say on which days a person, or a heavy load, may cross.
Pick a landmark for directions that the traveller cannot mix up
"Look for the cave near the big tree" is useless in a forest full of big trees, and the villager giving directions should know better. Here is how to choose, from everything near the target, the one landmark that is close and that no other landmark of its kind can be mistaken for.
Let hunted animals recover, unless too few are left to ever come back
Hunt a herd to a handful and it is back to full strength a week later, so the story's last white stag means nothing. Here is how to make a population slow to grow when small, doomed when too small, and how to avoid whole numbers that quietly stop recovery altogether.
Check a lever puzzle can be solved and find the fewest pulls
Pull one lever and two others flip as well; the door wants all three up. Some such puzzles have no answer at all, and the designer finds out when a player writes in. Here is a search over every possible setting that tells you at once, and gives the shortest solution.
Find lore facts that contradict each other across your world
The capital is a seaport in one chapter and three days from any coast in another, and each page looks fine alone. Here is how to keep lore as a list of small facts with a time span, so that real contradictions show up and rumours that are simply wrong do not.
Check every spell and power against the laws of your magic
You decided that magic always costs something and the dead cannot be raised, then forty spells later one of them does both. Here is how to write the laws of a world as rules a list of powers can be checked against, with the exact offenders named.
Scale a region's monsters to the player, then keep them from changing
If every region always matches the player's level, growing stronger means nothing, and if no region ever adapts, an early visit is hopeless and a late one is dull. Here is a middle way with a floor, a ceiling and a level that locks when the player first walks in.
Show the right moon on each night, even before day zero
A night scene says the moon is full, three nights later it is full again, and the players notice. Here is how to turn the count of days into a moon that waxes and wanes in the right rhythm, lists the full-moon nights in a stretch of days, and does not break for days before the start.
Close a mountain pass for winter, even when winter wraps around the year end
The pass is shut from late autumn until early spring, which means from day 300 to day 60 of the next year, and a simple range check says it is never closed. Here is the right test, how long to wait, and when a trip should set out to arrive in time.
Start a second playthrough with some things kept and the world reset
After the ending the player starts again with their best sword and some gold, but the world must be fresh. Keep the sword while the blacksmith who forged it has forgotten you, and the dialogue breaks. Here is a reset that keeps chosen things, caps what is too strong, and notices what depends on what.
Draw a found map with only the places that existed and that its maker had seen
A map found in a cellar shows the bridge that stood when it was drawn and not the mill built later, and it is wrong in just the way an old map should be. Here is how to build such maps from the world's own dates and the regions the mapmaker visited, with rumours for the famous places he never saw.