Here are my proposals for this thread.
First, errata Ring Duration: Ring (p. 112): Modify the first clause to read "until the target of the spell, or part of the target, moves outside the ring".
Second, errata Circle Target: Circle (p. 112): Add the following sentence as the third sentence of the definition. "Note that the circle itself is not inside the circle, and thus not targeted by the spell even if it would be a legal target."
Third, errata in the following insert (on page 113, although it wouldn't actually fit on there — this is a problem to be dealt with later).
Container Targets
Spells with "container" Targets, including Circle, Room, Structure, and Boundary, can work in one of two ways.
First, they can affect any valid target within the Target container at the time of casting, and continue to affect those targets even if they leave the original Target area, for as long as the spell lasts.
Second, they can affect any valid target within the Target container during the spell's period of effect. In this case, a valid target that leaves the container ceases to be affected by the spell, and a valid target that enters (or re-enters) the container is affected, until it leaves or the spell expires.
The way that a particular spell works is fixed when it is designed, and cannot be changed by the casting magus, although a spell working in one way is similar to a spell that is identical apart from working in the other, and so knowledge of one gives a bonus to inventing the other.
For example, a spell to put pink dots on people's foreheads with Target: Room and Duration: Moon could work in two ways. In the version that works in the first way, everyone in the room (on whom the spell Penetrates) at the time of casting gets a pink dot on their forehead, and this pink dot remains on their forehead until the new moon and full moon have both set. In the version that works in the second way, anyone who is inside the room (on whom the spell Penetrates) until the new moon and full moon have both set gets a pink dot on their forehead, even if they were not in the room when the spell was cast. When they leave the room, the pink dot disappears, although it reappears if they go back into the room. These two versions are two different spells, and a maga who wants to cast both (and cannot reliably cast them spontaneously) needs to create two spells.
Finally, make page 113 even more full by errataing in the following:
Circles and Rings in Three Dimensions
The standard Ars Magica rules use an intuitive definition of "inside the circle" for Circle Targets and Ring Durations. A person standing in the centre of a 1 pace diameter circle is inside the circle; someone who happens to be standing directly above it three floors higher is not. If your troupe need something more precise, you can work something out, and it should not break anything in the rules. Bear in mind, however, that it will still come down to the troupe's decision on whether a thing is within the more precisely defined volume.
Comments?