Thanks for all the great answers this week.
Maxime and Martin are correct, this falls under the upgrade clause of Deck Problem. Nemo has discovered that Amir is playing with a card in his deck which doesn't match his decklist, so this is a Deck Problem, and is upgraded since an opponent discovered it.
Let's talk about a couple of other options here.
First, Maxime points out that there isn't much room for advantage here. While this is true, not being able to gain much advantage in this specific situation is certainly outside of the Signifigant and Exceptional requirements for deviation.
I also want to discuss the opponent's great sportsmanship. Its possible the question didn't emphasize this enough. This situation actually happened at a large tournament a few weeks ago, and in that case Nemo really didn't want Amir to get a Game Loss. Nemo helped Amir find replacement cards, and even initiated the appeal of the initial Game Loss. Its tempting to say that this level of willingness to not have an opponent penalized warrants not giving the penalty. That considered, very generous sportsmanship still does not meet the requirements for deviation. We have even seen this at the highest levels, on Pro Tour video coverage, where very sporting players have attempted to decline penalties for their opponents and not succeeded.
Amir receives a Game Loss for Deck Problem, as his deck contained a card that wasn't listed on his deck list. The penalty is a Game Loss because it was discovered by an opponent.
Edited Joe Klopchic (Feb. 1, 2018 10:47:29 AM)