Some infractions in this document permit the judge to consider the possibility of a backup.
Due to the amount of information that may become available to players and might affect their play, 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.
A whole article could be written on this topic, and fortunately, one has been. In short, don’t be quick to backup. You need to take into consideration what actions the players have taken since, what information has been revealed, and the possibility of making things worse. We don’t always have to backup. It’s not “backup if you can, else leave everything alone” it’s “leave things alone unless it’s really really worse than backing up.” Remember, both players are responsible for the game state. No matter how messed up things have gotten, both players had opportunities to prevent it.
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.
This is a nice way of saying that the backup was minimally impactful. For example, the active player casts Terror on a White Knight, and the White Knight is put in the graveyard. In this case a rewind is quick and clean and doesn’t impact any decisions. But now let’s add to the scenario. Let’s say the active player cast the Terror to remove a blocker and then attacked with their entire team. In this case more decisions have been made. A rewind may be acceptable, but it may alter the attack. Finally, the active player cast Terror on the Knight, attacked. The opponent blocked. Combat tricks were used on both sides. In this case, too much information has been revealed. Backing up to the point of error would drastically impact the flow of the turn. The fewer decisions, the more likely a rewind is acceptable.
Only the Head Judge may authorize a backup. At large tournaments, they may choose to delegate this responsibility to Team Leaders.
This shouldn’t come as a surprise. Any time you are about to do something that cannot be easily reversed (such as an erroneous Game Loss, or a mangled rewind) the Head Judge must be consulted first. At big events, such as a Grand Prix or StarCityGames Opens, the Head Judge may allow Team Leaders to authorize backups. If this is the case, this will be communicated during the judge meeting. Now, this does not mean that if you feel a backup is a good idea, you can’t request a backup from the Head Judge.
To perform a backup, each individual action since the point of the error is reversed, starting with the most recent ones and working backwards. Every action must be reversed; no parts of the sequence should be omitted or reordered.
Ask the players what happened. You should have a clear understanding of each action that needs to be reversed before you start doing a backup. There should be no “I’m not sure what lands were tapped” or “I don’t remember if he attacked with that creature or tapped to use its ability.” Undo each action, in order. You don’t get to skip anything or switch stuff around. You must return exactly to the point of error, exactly the way it happened. This is to protect you as much as the players. If you rewind everything in reverse order, the players can see and understand what is going on.
If the identity of a card involved in reversing an action is unknown to one of the players (usually because it was drawn), a random card is chosen from the possible candidates.
Rewinding through a card draw can make people nervous. It’s really not that complicated, but still shouldn’t be done lightly. If backing up through a card draw, put the card back where it came from. If the specific card isn’t known to all players, put a random one back. Now, as a point of clarity, if you find yourself wanting to shuffle after returning a random card from a player’s hand as part of a rewind, you are probably doing more harm than good at this point with the rewind. Shuffling away a random card from a player’s hand is pretty much the definition of “disruptive”, but we will see below we can get around that.
Actions that caused a player to learn the identity of cards at a specific location in the library are reversed by shuffling those cards into the random portion of the library unless they were subsequently drawn; cards being returned to the library as part of the backup should not be shuffled at that stage if their identity was known to only one player.
If we are rewinding through an action that resulted in information about card location in the library becoming known (like scrying), we randomize the portion of the library that information was contained in (in plain language, we shuffle the previously random portion of the library, including the card(s) that became known). If we are rewinding through a drawn card, however, we don’t want to enable that card or another to be shuffled away, and so… it’s not. We simply return a random card from the hand to the top of the library.
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.
So, any time you are messing around with returning random cards from a player’s hand you run the risk of drastically changing the game. For example, the player has been holding on to a specific card since his opening draw. In a rewind, that card gets randomly put on top of the library. Then when we proceed forward, the opponent mills a card. Or we return a useless card to the top of the library, and they crack a fetch land to shuffle it away.
Some remedies state a simple backup may be performed. A simple backup is backing up the last action completed (or one currently in progress) and is sometimes used to make another portion of the prescribed remedy smoother.
Game Rule Violation and Hidden Card Error mentions that a simple backup may be done before applying a partial fix if it makes sense. This describes what a small backup is. Small backups are limited to one action, and exist just to make other fixes have intuitive results.
A simple backup should not involve any random elements.
If you are backing up through a shuffle or returning random cards from the hand to the library or anything of the sort, we are no longer in the realm of “simple”. Stop.