Low opl game 2 opponent forget to shuffle the trash to deck

Yes — this is a Deck Presentation Error (TR 703.4.a.4: incorrect deck size, e.g. forgetting to shuffle cards back into the deck between games), and at low OPL the fix is a Warning plus restoring the deck — no game loss, no rewind.

The remedy: have the player shuffle the cards still in their trash back into their Main Deck, verify the deck is legal size (40 in Constructed, TR 601.1.b), re-shuffle, present, and continue the game. Because the trash is public information and the only problem was a short deck (hand and board are untouched), restoring is easily possible — so even if the opponent (not the erroring player) caught it, low OPL policy says penalties are less harsh and this stays a Warning rather than upgrading to Game Loss (TR 703.1.c; FAQ #4469). The game just continues from the current state once the deck is fixed.