What is restoring game state and deck state

Restoring game state and deck state are judge remedies for fixing errors — not a single mechanic.

Restoring the game state means rewinding the game to the state before an error occurred. Each action since the error is reversed in reverse order (TR 701.5). A simple rewind (only the most recent action) can be done by any judge; anything bigger requires the head judge or designated appeals judges. Any card whose identity became known during the error (e.g. a wrongly drawn card) is replaced with a random card so no information advantage survives, and any known cards in a deck are shuffled back into the randomized portion.

Restoring the deck state means making a deck's contents legal again after a setup/presentation error (TR 703.4, Deck Presentation Error) — undoing sideboarding for game one, pulling a Champion that was shuffled into the deck back to the Champion Zone, or fixing an illegal deck size. The judge moves cards back to their correct zones (including swapping sideboard cards out of a hand for random main-deck cards) and shuffles the randomized portion. If the deck can't be restored, the penalty escalates toward a Game Loss. Decks must also be returned to their registered state before the first game of the next match (TR 403.8).