Question: What happens if the wrong wall is pushed out and someone notices and a player picked and racked their tile?

Answer:  If a player picks from the wrong wall and racks the tile, their hand should be declared dead. It is not written but since it says “is racked,” the intent here is that if it isn’t racked you can put it back.

Source:  Mah Jongg Made Easy 2024 Edition, pg. 19, (i)