Stepping on a crack in the sidewalk.
An old saying goes “Step on a crack, break your mother’s back.”
A crack in the sidewalk represented the opening of a grave and stepping om a crack meant inviting death to your family.
Walking under a ladder. A ladder leaning against a wall forms a triangle with the wall and the earth. According to traditional belief, the three sides of a triangle represent the family, so passing through the triangle destroys the wholeness of the family. That’s why people used to believe that walking under a ladder brings bad luck.