Dauntless Dismantler did nog destroy Iceberg Titan when sacrificed for X = 0 during a Premium Draft
While playing in a Premium Draft of Lost Caverns of Ixalan, I activated my Dauntless Dismantler to destroy two transformed artifacts, among which an Iceberg Titan which was attacking me for a lethal amount. Due to unknown reasons, the Iceberg Titan was not destroyed by the activation of my Dauntless Dismantler, and the Iceberg Titan hit my life total for a lethal amount after it resolved (unsuccesfully?).
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[Deleted User] commented
The backside of a transforming DFC has the same many value as the front. So X needed to be 2 to destroy the Titan.
202.3b The mana value of a transforming double-faced permanent or spell’s back face is calculated
as though it had the mana cost of its front face. If a permanent or spell is a copy of the back face
of a transforming double-faced card (even if the card representing that copy is itself a double-
faced card), the mana value of the copy is 0.