It can be a really big deal when someone casts magic.
As a Keeper, I usually ask them how they know the spell or how their casting it even before picking out requirements. Maybe Billy the Mundane read it on the back of a bathroom stall, maybe he's not sure what it does exactly (though his Player does, via the choices of the Use Magic list) and that sets up the fiction for if a miss or a glitch happens. Always play to find out what happens.
Sometimes when I ask this question the Player stops and thinks, 'Hm, maybe Billy really wouldn't know that' and finds another solution to the problem, otherwise in my games it's on them to find a reason and follow the fiction they establish.
If the same spell effect is sought out multiple times over a few stories or hunts, I usually codify it, writing down its requirements, the most common glitches or misses, and then adding it to a list of Custom Magic Moves, much like how the Hex Playbook codifies Rotes. It shows the Hunters know that spell, can teach it to their fellows and if they have what they need to cast it, can do so with fairly predictable results (glitches or misses as well).