Stepping on a Crack
There are some internet theories that this superstition actually had racist beginnings in the 19th century, but cracks have been something to avoid since far earlier.
European and Early American folk tales spread the notion that the empty space in cracks (whether in sidewalks, floors, walls, etc.) were actually connections from earth to the spirit world and messing with them in any way would cause trouble and misfortune.
This eventually gave way to the popular nursery rhyme, “Step on a crack and break your mother’s back.”