That certain types of rock and all cooked food look the same. Ironic, that. (tileset editing)
That no matter how light or heavy, only one object can ever be carried at a time, except for clothes, weapons, and such.
That rain causes seeds to self-destruct, leaving the dwarf carefully carrying it to become bewildered and confused for a few seconds.
That dwarves would much rather see puppies and kittens killed then lose something high quality they made.
That nobles will happily pull a lever right next to a floodgate while locked in their room.
That even the most hardy, stout, expert loggerdwarf even if very drunk will run at the sight of raccoons, deer, and anything else that moves, but will walk right up next to a unicorn or elephant and try to harvest a tree.
That expert trappers (supposedly the most outdoorsy of all dwarves) will knowingly and willfully wander onto an island when the river is frozen over, and sit there watching it thaw thinking nothing is wrong.
That humans seiging love campfires, but sometimes set themselves on fire from it, lighting up their whole squad - all while adjacent squads just watch and laugh - even directly next to a flowing river.
That even the dwarf that made the channel and pulled the lever will use any convenient channel to haul items around in, and not drop the stone they are carrying because of a little thing like not being able to breathe underwater.
That the more dwarves and animals a fortress has, the slower time moves.
That goblins, humans, elves, and any other sieging army will become utterly confused if they cannot find any open door to go through, and wait outside as if they rang the doorbell.
That unicorns are just pretty horses with a built in bayonett.
That dwarves like puppy and kitten leather items.
That statues can and do block even the mightiest flow of water or lava.
That swimming lessons and dwarves do not mix.