personally with a double 6 critical failure on the luck dice, I would maybe make them roll on the OOPS table and pay the stamina cost.
if the spell was to like boil water for soup I’d probably not do the OOPS roll, but if it was to like curse someone’s bloodline or make fire rain from the sky then they can roll oops too.
i kinda think of magic in Troika as like weaponized serendipity. how much are they asking of serendipity? is the question I ask myself in those sorta situations