Tournament rule 702.13 Facedown Card Error

Facedown Card Error [Warning]: A player adds a card to a facedown zone or removes a card from a facedown zone in a manner other than how they are entitled to under the Core Rules.

• 702.13.a Example: Hiding a card that does not have the Hidden keyword.
• 702.13.b Example: Moving a hidden card from a battlefield to a hand or Main Deck illegally.
• 702.13.c Remedies for illegally added cards:
• 702.13.d Remedies for illegally removed cards:

Official Tournament Rules