Once again your Monarch has mandated the impossible, they want a stone bed made of gniess for some inexplicable reason despite the protests of your best craftsmen and mason's that such a thing is impractical, unworkable and insane
Impossible demands will be highlighted in [!red with exclaimation points!], the monarch will be angry without someone to blame when the mandate comes to pass but it will be a notable event referenced in their reign, dwarves with comedic values will gain a positive thought about the absurdity of the situation and it may form the basis for what some may call the butt of jokes. Somehow achieving a impossible demand with a artifact (which is very slightly influenced between the period the mandate is set out) which will shower the monarch in a tantrum esque reset and counter-act the humiliation they might feel, will also add to the mythos of the artifact itself & be noted as a historical effect at the feat.
Its just a solution to the usually coddled lifestyle of mandate demanders with their never ending wants, and each impossible mandate is tracked as a historical instance for themselves and attributed to their reign to be ridiculed or praised, as set by personality traits like greedyness & ambition. Only dwarves and dwarven subjects in foreign nations are really able to pull it off without the prospect of looking like fools.
Hopefully as a result, silly demands can remain in the game even after they have been officially fixed, since its a known trait of the dwarven psyche's disposition for storytelling madness.