Andy and Nichole are playing in a Modern GPT. During his second main phase, Andy attempts to play a land for the turn. Nichole stops him, telling him that he’s already played a land for the turn. Andy says “Oops!” Then he puts the land back in hand and passes the turn.

Nichole untaps, draws, plays a land and casts Ronom Unicorn. Andy pauses for a second then says, “Hey. I went first, and now you have more lands in play than me. I actually didn’t play a land last turn.”

“Huh? … Whoa. You’re totally right.”

“Judge!”

What do you do?

Judges, feel free to discuss this scenario on Judge Apps!

[expand title=Answer]As many may have pointed out, this is a Communication Policy Violation.
Lets take a look at the following documents:-
Infraction Procedure Guide 3.7

A player violates the Player Communication policy detailed in section 4.1 of the Magic Tournament Rules (MTR).

Additional Remedy
If the situation is simple enough to safely back up without too much disruption to the course of the game, the judge may get permission from the Head Judge to back up the game to the point of the incorrect information. Each action taken is undone until the game reaches the point immediately prior to the error. Cards incorrectly placed in hand are returned to the location in the zone from which they were moved (if the identity of the incorrectly drawn card is not known to all players, a random card is returned instead). Once the game is backed up, it continues from that point.

MTR 4.1
Free information is information to which all players are entitled access without contamination or omissions made by their opponents. If a player is ever unable or unwilling to provide free information to an opponent that has requested it, he or she should call a judge and explain the situation. Free information includes:

• Details of current game actions and past game actions that still affect the game state.

The following rules govern player communication:

• Players may not represent derived or free information incorrectly.

Whether a player has played a land is free information, and Nichole has misrepresented that information unintentionally, so she gets a CPV Warning.

Now, let’s consider the fix. The Knowledge Pool Team are of the opinion that little enough has happened that the situation is simple enough to safely back up with minimal disruption to the game.

We rewind to Andy’s second main phase by returning Ronom Unicorn to Nichole’s hand, putting a random card back on top of her library, and tapping any lands that were tapped on the previous turn. Andy may now play his land, and the players continue the game from that point.[/expand]