Question: What happens if a player pushes out their next wall and picks from it before the previous wall was not completed?

Answer: If another player pushes out their wall while there are still tiles left from a previous wall and the player next in turn picks a tile from the wrong wall, both the player who pushed out their wall and the player who picked now have dead hands.

Source: Mah Jongg Made Easy 2024, page 20 #16(c)