Anya is playing against Natalie on day 1 of a Grand Prix. In game 2 of the match Anya activates her Cryptbreaker‘s first ability, discarding a card and saying “create a zombie.” She searches the Hour of Devastation Bundle box she is using for her deck box for a zombie token, and can’t find one immediately, so takes the contents out of the box and puts them on the table. She searches some more, finds the zombies, deploys them, then returns everything to the box. Returning to the game, Anya and Natalie both discover that Anya’s hand of 4 cards is missing. They call for a judge, and you find 19 cards in Anya’s deck box, separated in a group of 8 and a group of 11, divided by the tokens.
You investigate and confirm that Anya should have 4 cards in hand, based on the game state and number of turns passed.
What do you do?
Anya receives a Warning for Hidden Card Error. She put extra cards into her sideboard, and that action can’t be fixed using public information. Natalie will look at the 19 cards that were in Anya’s deck box and choose four to become Anya’s hand.
An expanded answer can be found here.