Did this happen between matches or not? You say he asked this outside of any matches, and then you say during a match. Did he notice this error during a match and then fail to call attention to it? How did he become aware of it? This sounds rather odd, of the “let's investigate” flavor.
In any event,
if this was discovered when our boy played his “Daring Thief”, when a player uses a checklist card to represent a double faced card, the double faced card is considered not to exist for the purposes of deck legality.
Why do you say that the penalty for using the incorrect checklist card would be a Warning? I can't find anything in the IPG to support this conclusion.
We generally correct the list to match the deck, but here we have three sources of information: The marked checklist card, the double-faced card, and the decklist. In this case, I'd support correcting the checklist card (and decklist, if necessary) to match the double-faced card, as that seems to more accurately reflect the player's intent.
In any event, this deck does not match the decklist, so this is unambiguously a Game Loss.
However,
if this was discovered outside any match, there's no infraction being committed. We can't issue a Game Loss for something that happened last round. We just need to verify that the player's decklist is legal and direct them to ensure that their deck matches their list. That will involve them finding a correct checklist card and adequately marking it before their next match.
Edited Dan Collins (April 24, 2016 09:50:03 AM)