Anna and Nadine are several turns into their game at a local Legacy GPT. Anna casts Swords to Plowshares targeting Nadine’s Deathrite Shaman, which she cast on her previous turn. Nadine gains 1 life, and Anna passes the turn. Nadine untaps and Anna sees that Nadine put the Shaman in her graveyard instead of exiling it. She then notices that not only is that one in the graveyard, but the Shaman that she exiled way back on turn 1 is in the graveyard too. Anna raises her hand and calls you to the table.

What do you do?

Judges, feel free to discuss this scenario on Judge Apps!

When each Deathrite Shaman was put into the graveyard during the resolution of Swords to Plowshares, the players violated the game rules. Specifically, an object changing zones moved to the wrong zone. And the controller of the Swords was not the player that carried out the zone change. The IPG specifically calls out actions to be taken under these circumstances.
Originally posted 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 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.
Each player receives a Warning for a Games Rules Violation. In addition, we will exile both Deathrite Shamans from the graveyard. The phrase “without disrupting the state of the game” refers to the current game state and should not take into account any intervening consequences of the error, such as Tarmogoyfs that had been given +1/+1 by the Deathrite during previous attacks. If cards that care about the contents of the graveyard are currently on the battlefield, some judgement may be required regarding whether to exile the Deathrite that died several turns before. However, in most situations there will not be an immediate, game-changing impact due to exiling the Deathrite and it should be moved to the correct zone.

Finally, we know that the error occurred twice. However, IPG 1.3. Applying Penalties tells us “if the root cause is the same, only the more severe one is applied.” In this case, the root cause is incorrectly resolving Swords to Plowshares. Since the errors were identical, neither is “more severe” and a single Warning is issued to each player.