This came up in yesterday's Modern WMCQ that inspired some extensive discussion between judges.

Alan casts Ulamog, the Infinite Gyre. Nancy casts Remand in response. Alan, thought that his spell has been countered, put Ulamog into graveyard, then resolved Ulamog's shuffle graveyard into library trigger and shuffled Ulamog and another card into the library. Shortly after, they found the mistake and called a judge.

The judge ruled GRV for both players, and he decided to backup by searching Ulamog and the other card (which both players can confirm) out of the library, shuffle that library, put back a random card onto Nancy's library from hand, then put Remand and Ulamog back onto stack resolving.

This seems to be an acceptable fix for both players. Now let's extend the situation. Let's say at the point of error, Alan has about twenty cards in his graveyard, which both players can't either confirm the number not the content of those cards. As long as a full backup is impossible to perform, what do you do in this situation?

There would be no way for a full rewind so I would leave the board state as is and remind both players to be more careful in the future. Everything (including Ulamog) will remain in the deck and Nancy will draw a card off remand. I would issue a GRV for Alan and a FtMGS for Nancy.

I agree with Brock that if a full backup is impossible, we have no choice but to leave the game state as it is (as the current game state is incorrect but legal). However, I would issue a GRV Warning to both players, as per IPG 2.5:

If a player takes an action called for by an effect controlled by his or her opponent, but does it incorrectly, both players receive a Game Play Error – Game Rule Violation. For example, if a player casts Path to Exile on an opponent’s creature and the opponent puts the creature into the graveyard, both players receive this infraction.

In case of a full backup is impossible, is it allowed to apply the following partial fix as stated by IPG 2.5?

If an object changing zones is put into the wrong zone, the identity of the object was known to all players, and it can be moved without disrupting the state of the game, put the object in the correct zone.

If this partial fix is applied, we search Ulamog (but nothing more) from library, shuffle the library, put it back into Alan's hand (as if Remand is correctly resolved).

Originally posted by Hao Du:

In case of a full backup is impossible, is it allowed to apply the following partial fix as stated by IPG 2.5?

If an object changing zones is put into the wrong zone, the identity of the object was known to all players, and it can be moved without disrupting the state of the game, put the object in the correct zone.

If this partial fix is applied, we search Ulamog (but nothing more) from library, shuffle the library, put it back into Alan's hand (as if Remand is correctly resolved).

I Don't think the partial fix could be applied here since Ulamog was shuffled back into library due to it's trigger, another ability and was not part of action when trying to resolve Remand, which would be treated same as the other card which also known to all other players that shuffle back to library with Ulamog.

Ulamog was put into the graveyard from resolving Remand incorrectly - that itself fits the clause of the basic zone change remedy. The fact that it's now part of a randomized library doesn't really matter, IMO - we know where Ulamog is and we can find it without disrupting the game further (the library is already randomized, so any ordered information is gone), so I don't have a problem doing so.

The other cards that may have been shuffled into the library are an unfortunate side effect of this error, but cannot reasonably be restored according to policy, in my opinion.