I'm pretty sure the thing Mal buried was her totem--the spinning top thing. She left it lying down, symbolizing either a) Limbo is reality or b) She doesn't want to know if limbo is reality or not.

When Cobb breaks into her safe, he spins the top and then closes the safe again--thus instilling the belief in her that her world isn't real. That one simple action is probably what caused so many problems later.