The botch is in InVi, so anything "Muto" related would not, imo, be approp.
A couple thoughts - one, I like to personalize such virtues, to the magus/maga in question. What other virutes/flaws do they have? What is their "theme", if any?
Then, the actual spell effect that triggered the Twilight can be considered - spin off of that, and it's more personalized to the event, rather than a "generic" InVi result like a (second?) focus or whatever.
Lastly, I like to go outside the book - rather than go shopping down the virtues, I'd rather create something unique, based on the event and this magus, as Twilight effects (imo) should only rarely be duplicated between magi or events.
So... how about a Power? Come up with an InVi effect that the mage can use "at will" (similar to that part of the Mythic Blood virtue), or, if the effect is truly powerful, at the cost of one fatigue. InVi effects are often not very useful below 7th mangitude or so, and are broken up into many sub-spells - for items, for effects, for creatures, for each Realm within that, for vis, for regios - by the time you're done, you'd need scores of different spells to cover every corner of it, and that's before you start to get extra magnitudes for extra information. So giving them one will hardly dent the InVi depths (and might add an information spell to make later stories flow better!)
Take the spell "effect" s/he was casting, modify it to the appropriate magnitude (SG/Troupe judgement - see "similar" spell, but don't be a slave to it), and turn it into a Power - one fatigue, go crazy. Maybe a narrow group of effects, if you want a truly impressive Power.
Or, if you want to mess with them, give them something "small" - too small, like some petty minor virtue. PLUS a "Latent Magical Ability" that will only be noticed later - something that works automatically, or randomly until they get control of it (RP'd, or sim to an Ability).
(Level 11 botch - love to see the look on that face!)
