702.13. Nonpublic Card Error [Warning]: A player interacts with a nonpublic card in a manner other than how they are entitled to under the Core Rules.
702.13.a. This includes hiding a card that does not have the Hidden keyword.
702.13.b. Upgrades to [Game Loss] if a player hides a card that doesn’t have the Hidden keyword and doesn’t correct it before it has an impact on an opponent’s action.