I think Andre's answer is correct (except that the infraction is known as GPE-GRV): there are far too many decision points passed to allow for a full back-up, and the partial backup explicitly allowed under the fixes for GRVs does not help, as the Snapcaster Mage is no longer in the wrong zone. There is, however, one wrinkle to this question which may be worth exploring.
EDIT: Scott Marshall has clarified that the following point is not relevant for the purposes of this discussion.Originally posted by Dustin De Leeuw:
In his precombat main phase, Anthemius casts Path to Exile on Numerian’s Snapcaster Mage, which he puts into his otherwise empty graveyard.
What does this phrase mean? If the player's graveyard and exile zone are empty, how do we distinguish one from the other? As far as I know, there are no rules which prescribe a physical space in the play area for either of the zones, and I have witnessed players adopt all sorts of different conventions in practice. Assuming N's exile zone was empty at the time of resolving Path to Exile, how then do we determine whether in fact he put the Snapcaster into the graveyard or into exile? (I am aware that, technically, the exile zone is a shared zone, so from that point of view it does not make sense to talk about N's exile zone. In practice, however, players act as though each player had their own exile zone, each keeping their own exiled cards separate from their opponents' cards.)
Is the player's subjective intention enough? I don't think so: as a matter of practice, when we look to see what zone an object is in, we don't consider what zone the player intended it to go to, but rather we look to see where the object is physically located in relation to other objects and classify it accordingly. In this case, however, there are no other similarly located objects, as the graveyard was previously empty.
Can we then look to the players' subsequent conduct regarding the object? I think ultimately that this is the correct test as it is the most practical in application: if both players are treating a face-up pile of cards as player A's graveyard, then that is what it is. The problem here is that, even if N subsequently treated the Snapcaster Mage as being in his graveyard, it is not clear that A did so. A only pointed out the problem shortly after N used his Moorland Haunt—in other words, shortly after it became relevant which zone the Snapcaster mage was in. To me this is consistent with A treating the Snapcaster as being in exile up until that point—after all, until then he had no reason to assume that the Path to Exile had been incorrectly resolved. This will depend, of course, on the circumstances of the match as a whole: if, for example, N had in previous games used a particular physical space for his graveyard and in this game put the Snapcaster in that space, then it is fairly clear that the card was put into his graveyard. If, on the other hand, this is the first game between these players, then I think that there is a reasonable argument to be made that the Snapcaster was in fact exiled during the resolution of the Path to Exile, and that the mistake occurred later when N incorrectly paid the cost of Moorland Haunt's activated ability by attempting to exile a creature card which was already in exile. If that is the case, then we are still in the realm of GPE-GRV for N, with a GPE-FtMGS for A (as he didn't speak up until other game actions had occurred), but the fix is different: in this case, it is quite simple to back up to before the Moorland Haunt activation, and I would seek permission from the Head Judge to do so.
Edited Robert Hinrichsen (Nov. 28, 2012 07:46:59 PM)