When using the deckbuilder it is possible to add a card called "Card not found" to the deck. In order to do so, at least one card must've been selected, before clicking an empty space in the card selection window. This will add a card called "Card not found", with rarity and mana cost matching the card selected prior to its addition. As well as this, it will add an "undefined" to the hyperlink of the pre-saved deck, which will cause the error message "Server Error (500)" to be displayed after saving the deck. The deck will still be saved, but the "Card not found" card is removed during that process.