How should a judge handle a situation where a player accidentally shuffles their chosen champion into their deck after mulligans are complete?

Ruling: This is a Deck Presentation Error. If the erroring player catches and reports the mistake themselves, issue a warning, remove the chosen champion from the deck, and place it in the chosen champion zone. The game continues without rewinding the mulligan.

Sequence:

• Isolate the mulliganed cards (ask both players how many were mulliganed, pull that many from the bottom of the deck)
• Remove the chosen champion from the remaining deck
• Shuffle the rest of the deck
• Place the shuffled deck on top of the mulliganed cards
• Place the chosen champion in the chosen champion zone

Nuances:

• If any other player (including opponent or spectators) catches the mistake instead of the erroring player, it would be a game loss at high OPL (but still just a warning at low OPL)
• If the player runs multiple copies of their champion in the deck, you must check their hand and remove any champion copies from hand, shuffle them back into the deck, then draw replacement cards after placing one in the champion zone
• At low OPL without deck registration, verify the player doesn't have another copy in their deck or hand before proceeding