What we have here is the rare and elusive Tournament Error – Communication Policy Violation. Although Natalie is not required to tell Aldis the power and toughness of her creature, she is also not allowed to get it wrong if she chooses to answer. So, we we have a Warning for CPV, but what else are we going to do?

CPV supports backups, but to which event should we back up? The IPG says

IPG 3.7 Additional Remedy
A backup may be considered in cases where a player has clearly acted upon incorrect information provided to him or her by his or her opponent. The backup should be to the point of the action, not the erroneous communication.

So, right off the bat we know that we will not be going back to the point of the incorrect answer. This leaves us two potential actions, which may have been based on erroneous information: attacking with the Behemoth, and casting Complete Disregard. While the decision to attack was likely informed in part by the power and toughness of the Warleader, it is not clear that correct information would have had any impact on this play. The choice to cast Complete Disregard with an illegal target, however, was unambiguously the result of thinking the Warleader was 3/3. Since very little has happened since that decision, we simply return the Warleader to the battlefield and back up the casting of the spell. Since Aldis’s error was a result of Natalie’s CPV, we do not assess him a Game Rules Violation.