Failure to Reveal is a game play error that applies when a game rule or an effect has instructed a player to reveal some information and it is not revealed at the correct time. In most cases this involves revealing a card to prove that the card has specific characteristics. For example, when resolving Expedition Map‘s ability you reveal the searched for card to prove that it was a land card that you retrieved. Somewhat similarly, you reveal all face down cards at the end of a game to prove that they have morph.
The penalty for failing to reveal a card at competitive and professional REL events is a game loss. If the card is identifiable; for example you have no cards in hand and searched for a card, then this penalty can be downgraded to a warning at competitive events. This penalty is normally a game loss due to the potential for advantage or abuse is fairly high.
Just as it’s important to point out what failure to reveal is, there are common misconceptions about the penalty and things that failure to reveal is not. For example, If a player misses the triggered ability of Dark Confidant and draws their normal card for the turn, they are not failing to reveal as they weren’t resolving Dark Confidant’s ability. The appropriate penalty in this situation is Missed Trigger, not failure to reveal. It is especially important for judges to try and make sure the appropriate penalty is applied in situations like these.