Thanks to everyone for the discussion - let's see how this all played out…

With the Head Judge's permission, Chris would receive a Warning for a downgraded Deck /Decklist (D/DL) Problem. While Chris has presented an illegal deck and made a mulligan decision with that deck, he has called this error on himself and has not had any opportunity to gain any advantage from the situation. Two things are important to notice here - first, as per the Magic Tournament Rules, “The game is considered to have begun once all players have completed their mulligans”. This is how we are establishing the “has not had any opportunity to gain any advantage from the situation” - had mulligans been completed and this issue been discovered any time after that, such as while performing some sort of search / tutor effect, the downgrade clause would no longer apply. Second, we are not forcing any mulligans on Chris - this is not a Improper Draw at Start of Game - he was in the middle of resolving his own mulligan when he discovered a D/DL Problem - we apply those fixes to this situation (find the missing card) and carry on from that point - Chris would continue with his mulligan to 6 cards.

Paul will receive a Game Loss for his D/DL Problem. As noted previously, he had very obviously started his game and had not called the error on himself, so neither of the prerequisites for the downgrade clause can be applied here. As opposed to when a larger number of D/DL Problems are discovered (during beginning of round deck checks), both players in Paul's match are allowed to sideboard before game 2, as a (partial) game of Magic has actually been played.

Thanks again, and we'll see you at the next Knowledge Pool!