What is the most irrational superstition you have heard about?

My grandmother was the superstition queen, no hat or shoes on the bed, stop a rocking chair when you get off of it, step on a dropped object twice before picking it up, say bread and butter if walking and someone passes between you, salt over the left shoulder, black cat crossing paths, stay in bed on friday the 13th, do not break a mirror, cover mirrors during wakes, do not step on cracks, do not go out during full moons, stay out of graveyards at night, and so many others I cannot remember.