(blorb kinda means "the prep for the game is fundamental, improvising is what you do only when your prep has failed you and hopefully you don't have to do it, also prep things not events". If we have a nice canonical description give it to me so I can link it thanks.)
Here are a couple scenarios that my inner @2097
didn't know how to evaluate.
(a) Prep says "under the cloth is a cursed mirror which [does bad thing upon looking in it], but it's also really old and the curse may have lost its efficacy. Roll 1d6 and on a 6 the mirror is just a mirror". Players feel under the cloth, feel glass, assume it's a cursed mirror, take it with them, show it to an enemy, GM rolls a 5, enemy gets [bad'd].
(b) Same as above, but GM rolls upon entering the mirror room and knows it's a 5 the whole time the players have it with them covered.
(c) Prep says "it's either a cursed mirror or a steel plate with [info] painted onto it. When the players do something that would tell the difference, roll 1d6 and if it's a 6 then it's the steel plate. Please describe it in such a way that it's ambiguous as long as plausible, for example if they just reach under the cloth and feel it rather than looking."
(d) Same as (b), but prep says "GM, identify the character or characters with approximately a 1/6 chance to be the first to interact with the mirror. If multiple, choose between them randomly. Write this character down. If that character is indeed the first to interact with the mirror, it is not cursed."
(e) Same as (c) but with the which-character-first-interacts mechanics.
How blorby do those feel? Completely blorby? Compromised in some way?