MutoVim on spontaneous spells

Under the guide lines for Muto Vim it says that you may not target spont spells

In the mysteries revised, names of power allows you to target spont spells.

Is this an error or are we just saying "its a mystery it can do as it wishes"?

that's the cool thing about names of power, it's also the cool thing about consummate talisman.

Being able to use names of power on spontaneous spells is what the mystery is about. It's the same issue as philosophic alchemy being able to produce vis that isn't vim vis. That's the whole point of it.

Mysteries are a little bit odder than other Virtues, but the core rule seems to be "something that requires a Virtue can do 'a bit more' than normal magic, and a Major Virtue allows more than a Minor one". Even the "standard" Hermetic Virtues boost or break a few normal casting formulae or restrictions...

Note, though, that the TMRE MuVi Mysteries don't go as far as you suggest - you can not target Spont. Spells:
instead, they allow an unusual bonus - they target the Casting Score of the casting magus - with a note that a Mom. Duration in this case affects the casting Score of the immediately next spell cast (ie in the "next round"().
Because it affects this unusual property, and affects the magus himself, it allows the magus to cast a Spont Spell. (And the bonus is divided by 2 or 5 when you cast the Spont spell, to get the Casting Total from Casting Score)

Standard Hermetic MuVi does not affect the Casting Score, and standard MuVi requires simultaneous casting with the affected spell (not "next round").

The Consummate Talisman's special kick is that the MuVi effects can targets the magus' Casting Score - normally a MuVi effect in a device can only affect other enchanted effects in the same device.

I think it says you cant effect spontaneous spells because spontaneous spells are too fragile and any attempt to mess with them would cause the original spell to fail or unfortunate things to happen. So i would think using MuVim, you could counter any spontaneous spell cast by messing up the spell.