Ill-fortune is said to be the result from stepping on a crack in the pavement. Present day society usually associates the superstition behind treading on cracks to the rhyme: "Step on a crack, break your mother's back" but the superstition actually goes back to the late 19th - early 20th Century and the racism that was prevalent in this period.
The original rhyming verse is thought to be "Step on a crack and your mother will turn black." It was also common to think that walking on the lines in pavement would mean you would marry a negro and have a black baby. (Apparently this superstition only applied to Caucasians and because of the rampant prejudice against black people, was considered an activity to avoid.)
Stepping on cracks also had significance for children. In the mid-20th Century it was popular to tell children that if they stepped on the cracks in the street, they would be eaten by the bears that congregate on street corners waiting for their lunch to walk by.
Also, the number of lines a person would walk on corresponded with the number of china dishes that the person would break, later in the day.
Only in the last few decades has the rhyming superstition resurfaced to be the recognized "step on a crack, break your mother's back" and in some areas, two superstitions above are melded together to include the number of lines one steps on will correspond with the number of your mother's bones that are broken.