There are limitations to searching your Deck, and as such players must know all the cards left in their Deck just by what is on the field, in their Graveyard, and in their Hand. There are two types of search effects.
Card effects that require you to look through your Deck for cards have no real rules; If the effect resolves, you must do it. However, if upon your search you find that no card exists that meets the search criteria, your opponent will have an opportunity to look through your Deck and verify that there is nothing to search for. After they have finished, shuffle your Deck.
- Example: "Sangan" on the field is destroyed by "Dark Hole" and sent to the Graveyard. The owner of "Sangan" must search their Deck for a monster with 1500 or less ATK. The owner cannot find a monster in their Deck with 1500 or less ATK, so the opponent looks through it to verify.
Card effects that you choose to activate will require you to know your Deck inside and out. You are not allowed to activate a card effect to search for a card in your Deck if you know that all the cards that meet the search criteria are no longer in your Deck. To do so, then find there's nothing to search for and shuffle the Deck is illegal, you are not allowed to shuffle your Deck unless you are required to by a card effect. This is a form of stacking the Deck and is considered cheating.
- Example: A player controls a face-up "Overdrive Teleporter". That player runs 3 copies of "Psychic Commander" and 2 copies of "Mind Protector" in their Deck. That player searches their Graveyard, finding 2 "Psychic Commanders" and 1 "Mind Protector". That player also has 1 "Psychic Commander" in their hand. "Overdrive Teleporter's" effect cannot be activated, because "Overdrive Teleporter's" effect Special Summons from your Deck 2 Level 3 Psychic-type monsters, and the controller found 4 of their 5 existing outside of their Deck.
While a player will usually know their Deck inside and out, there are those who will make a mistake and activate an optional search effect without realizing their search will not succeed, known as an Illegal Activation. In this situation, the opponent may verify the Deck, the Deck is shuffled, a warning is given to the player who activated the card, and the gamestate is rewound to before the activation. If this was done because of a Spell card played from the Hand, the card is NOT sent to the Graveyard, but is instead returned to the player's Hand.
- Example: A player activates "Reinforcement of the Army" from their Hand. Upon searching the player finds there are no Level 4 or lower Warrior-type monsters remaining in the Deck. The opponent checks the Deck to verify, then the Deck is shuffled and "Reinforcement of the Army" is returned to the player's Hand. The player receives a warning and play resumes from that point.
Legal Activation vs Illegal Activation
There are situations where it is legal to activate a card where, when it resolves, there are no viable targets in the Deck to search for. This would normally be due to the Cost of the card or the beginning of its effect, or another card in the Chain. This results in the resolution of the card, the optional verification of the opponent, and no penalty given to the player who activated the card.
- Example: A player activates "Charge of the Light Brigade" with the knowledge that only 1 Level 4 or lower "Lightsworn" monster remains in their Deck. That monster is sent to the Graveyard as part of the Cost of "Charge of the Light Brigade". The opponent may verify, then the Deck is shuffled and play continues normally.