I had recently following situation in a Standard Tournament at REL Competitive:

Adrian attacks Newton with an animated Shambling Vent and a Siege Rhino, Newton is at 5 life and has no blockers, so the attack is lethal. He fetches a Prairie Stream and casts and resolves a Crackling Doom. Adrian passes the turn with two cards in hand and four untapped lands. Newton draws for his turn and plays a Fetchland, fetches and calls you over, because he just realizes that he has only one basic land in play, so the Prairie Stream should have come into play tapped.
You are sure that there is no cheating involved.

Penalties are pretty obvious, but the main question: Do you back up or not? Why do you and why don't you back up? Would it be different if Adrian's attack wasn't lethal? (It should not be, but is it really?)

I talked to a few judges about that and got many different answers, so I was asked to post it here.

Thanks for your answers :)

Edited Valentin Hauser (March 22, 2016 11:56:28 PM)

Originally posted by MTR 1.4:

Backups involving random/unknown elements should be approached with extreme caution, especially if they cause or threaten to cause a situation in which a player will end up with different cards than they would once they have correctly drawn those cards. For example, returning cards to the library when a player has the ability to shuffle their library is not something that should be done except in extreme situations.

-and-

A good backup will result in a situation where the gained information makes no difference and the line of play remains the same (excepting the error, which has been fixed). This means limiting backups to situations with minimal decision trees.

All of that suggests - quite strongly, in fact - that we shouldn't even think about a rewind in this situation.

However, there's a phrase in that first quote - “except in extreme situations” - and an additional quote to consider:

backups are regarded as a solution of last resort, only applied in situations where leaving the game in the current state is a substantially worse solution

You mentioned that investigation leads us to conclude honest mistakes, but our investigation would also inform us as to whether or not Newton had any other way to prevent the lethal combat damage. If his options were to play Crackling Doom (illegally), or lose the game, then I'd zip through a (mostly irrelevant) rewind and let the game end the way it should have. After all, leaving it as is seems substantially worse.

d:^D

Edited Scott Marshall (March 23, 2016 12:26:19 AM)

At first glance what I'm thinking it's don't back up, because there's information that has been revealed and there's a fetch involved in this equation.

If we still considering a backup anyway, the complete backup should be:

• Take the land fetched by Newton and shuffle it in the random portion of deck.

• Take the fetchland in graveyard put it there by Newton and put in play.
  

• Add 1 life to Newton life total.
  

• Take a card at random from Newton hand and put it in top of library.
  

• Go back to Adrian turn.
  

• Going back to combat, delete the combat damage.
  

• Move back to declaring attackers after the attackers has been declared
  

• Add 2 lifes to Adrian life total.
  

• Put back the creature sacrified by Adrian in play and attacking.
  

• Put the Crackling Doom from Newton Graveyard in his hand.
  

• Untap the lands used to pay the card leaving the Prairie Stream tapped.
  

• Continue the game from here
  

I had marked the two first steps because, these are the potential steps were cards in a known place can be lost due a voluntary deck shuffle by his controller.

Considering this is why I don't make the backup and be leaving the game as it is right now.

I'll be glad to read other answers or comparisons between what I said and what must be do.

You must return the Fetchland to Newtons hand and Adrian loses 2 life (Shambling Vent has lifelink) just to be super correct, but I get your point :)

Originally posted by Valentin Hauser:

You must return the Fetchland to Newtons hand and Adrian loses 2 life (Shambling Vent has lifelink) just to be super correct, but I get your point :)

I end my comment after Scott post it, so i didn't read it.
Consider what he say at the end.

Scott Marshall
You mentioned that investigation leads us to conclude honest mistakes, but our investigation would also inform us as to whether or not Newton had any other way to prevent the lethal combat damage. If his options were to play Crackling Doom (illegally), or lose the game, then I'd zip through a (mostly irrelevant) rewind and let the game end the way it should have. After all, leaving it as is seems substantially worse.

d:^D

As always, im glad to be helpful.

This situation actually came up during a side event at GP DC. A player discovered that he and his opponent had forgotten the +1/+0 that went with the intimidate on an activated ability roughly a turn and a half before. There was no CPV, and the defending player was left at “1” due to the forgotten pump.

I elected to rewind through a bunch of individual actions that I normally wouldn't because the game was actually over, so there was no way for subsequently revealed information to damage it.