You are the only judge at a local Modern GPT. Nat calls you over and explains that Addison just cast a Tribal Flames on his Siege Rhino, saying “5 to your rhino.” Nat responded by activating his Tectonic Edge targeting Addison’s Blood Crypt. The players had then both recalculated for Tribal Flames, and realized that Addison had only controlled 4 basic land types before Nat activated Tectonic Edge. You believe that Addison’s mistake was unintentional. What do you do?

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[expand title=Answer]As you all can tell, Addison misrepresented how much damage her Tribal Flames would do when she cast it. Addison is therefore guilty of a Communication Policy Violation. Look at a parallel situation: We give a CPV if you say ‘attack you for 3 with my Tarmogoyf’, then realize you accidentally miscounted when no blockers are declared and deal 4 damage. Just like with a tarmogoyf miscount, you also can’t give inaccurate information about what a spell will do when it resolves. For her infraction, Addison will receive a warning. As for the fix, CPV does give us the ability to back up, and this is a situation where the gameis in a significantly better place if we back up than if we do not. Since it occurred after the CPV, the Tectonic Edge activation will be undone, and the Blood Crypt returned to the battlefield. Since casting Tribal Flames was legal, that action will stand. Play will resume with Tribal Flames on the stack targeting Siege Rhino, and Nat having priority.[/expand]