What is the penalty for lying about a face-down card?
Cheating is not tolerated at any REL and must always result in disqualification.
The current Rules Enforcement Levels (REL) are Regular, Competitive, and Professional.
The Tournament rules state about cheating:

5.1 Cheating
Cheating will not be tolerated. The Head Judge reviews all cheating allegations, and if he or she believes that a player has cheated, he or she will issue the appropriate penalty based on the Infraction Procedure Guide or Judging at Regular REL document. All disqualifications are subject to DCI review and further penalties may be assessed.

The rules for Judging at Regular REL state:

Serious Problems
Certain actions will not be tolerated under any circumstances. Every effort should be made to educate players before and during events; however, ignorance is not an acceptable defence of these actions. Any player engaging in the following must be removed from your event and, at the Organizer's discretion, removed from the venue entirely:

Aggressive, violent or abusive behavior (physical or verbal).

Intentionally and knowingly breaking or letting an opponent break game or tournament rules, or lying. (“Bluffing” about cards opponents can't normally see is permitted).

Determining match outcomes by incentives, coercion, or outside-the-game methods, or gambling on any part of a tournament.

Theft (including things like replacing a card in a draft with one from a player’s binder).


Removing players in this way is called a Disqualification, and we must always try to educate our players on why these actions are unacceptable. Also let the player know that while your decision is final, the Judge Program would still like to hear his or her side of the story. You can contact your local Regional Coordinator, high level Judge, or WPN Representative to guide you through the process of a Disqualification.

The Infraction Procedure Guide is used for penalties at Competitive and Professional REL and states:

4.8. Unsporting Conduct — Cheating
Definition
A person breaks a rule defined by the tournament documents, lies to a tournament official, or notices an offense committed in his or her (or a teammate's) match and does not call attention to it.
Additionally, the offense must meet the following criteria for it to be considered Cheating:

The player must be attempting to gain advantage from his or her action.

The player must be aware that he or she is doing something illegal.


If all criteria are not met, the offense is not Cheating and should be handled by a different infraction. Cheating will often appear on the surface as a Game Play Error or Tournament Error, and must be investigated by the judge to make a determination of intent and awareness.
Penalty: Disqualification