My ruling on it would be that disrupting the ritual circle should cause an automatic failure on the caster's discipline roll for that round. Forcing them to choose to take the backlash to continue the spell (at least long enough to release the energy safely), or to release the energy as fallout.
The fallout could place the aspect of 'uncontrolled magic' on the scene, which could then be invoked by any caster present to aid their own spells.
Having said all that, as shown in Changes, it's possible for a caster to cast a thaumaturgy spell entirely within their own mind, but that takes time and precision. This does theoretically mean that a disciplined caster could continue with and complete the spell after the circle was broken, if given the chance to do so (i.e. not pounded into the ground by a rampaging PC).
My ruling on it would be that disrupting the ritual circle should cause an automatic failure on the caster's discipline roll for that round. Forcing them to choose to take the backlash to continue the spell (at least long enough to release the energy safely), or to release the energy as fallout.
The fallout could place the aspect of 'uncontrolled magic' on the scene, which could then be invoked by any caster present to aid their own spells.
I like this idea quite a bit.
Having said all that, as shown in Changes, it's possible for a caster to cast a thaumaturgy spell entirely within their own mind, but that takes time and precision. This does theoretically mean that a disciplined caster could continue with and complete the spell after the circle was broken, if given the chance to do so (i.e. not pounded into the ground by a rampaging PC).
This I don't agree with, though.
The spell in question was, from the ground up, a purely mental construct. If the spell is designed that way to begin with, all well and good. If it's designed to use a circle, implements, and other ritual aids and then that physical construct is disrupted... I personally wouldn't let a caster switch it to purely mental mid-ritual unless it was something really simple. (Like caster's Lore rating or less.) The difference is that one was designed to be purely mental -- Harry's Spell -- and one was designed with a circle and that 'crutch' was suddenly broken.
In the case of someone breaking a circle during a significant ritual, not only is there the possibility that the caster(s) concentration could be disrupted, but also that the 'background' energy or magic in an area could start flowing into and becoming part of the power gathered for the ritual. This could result in the ritual having more power than the caster(s) wanted, or could even control. It is also possible that depending on the nature of the magic or energy, it could effect the results of the casting.
In Turn Coat for instance: (Spoiler alter)When Harry was doing the sanctum invocation ritual at Demonreach, after he drew the circle, but before he empowered he, he stripped completely except for his pentacle. It was mentioned that he did this not so much because the ritual required it, but to reduce the chances that any of the enchantments on his gear could cause interference. Granted, a sanctum invocation is apparently not a very common, or safe ritual, particularly when cast at a location as friendly as Demonreach, it does make it seem that for delicate or high energy castings, even a little errant magic can have a major (negative) impact.