The usual suspects, pee stored in the balls, cooties, etc. But some dumb beliefs will outdumb them all:
I used to own a plastic orb that when pressed would activate a light inside it and keep it glowing as long as you would squeeze it as one would expect a crystal ball would glow. I wasn't allowed to turn the lights on after 10, a power-saving thing my parents believed. Ever since I was read a particular story a fear of what hid in the darkness filled every inch of my mind whenever I had to go to the bathroom at night, and until I got that little ball I would rather hold it and hope no accidents occurred until morning than brave the hallways. I believed back then that for as long as I kept the ball's light going that I would be protected from the shadowy beings that would otherwise assault me.
So I squeezed it as hard as I could with as much bravery as I could muster with every step, truly in belief that it would help against phantoms that did not exist on my epic quest through the many-cornered hallways of my family home. For a time I was fine. Little did my young self know that, eventually, the orb's batteries would run out. One night it did finally give out just as I reached the bathroom door. Needless to say I panicked, too choked with fear to make much more noise than frantic whimpering as I begged the little ball to shine once more, and eventually just resigned to whatever demons might fall upon me.
Nothing came, it was just me in the dark, with a ball that won't shine anymore. It was quiet, it was peaceful. Business was concluded as usual but without my faithful little light. Today I treat the dark with a healthy sense of paranoia, as people should. But to fear it so that you're paralyzed by its presence without a crutch to abate it? Not anymore.
Dead belief - I'm a wizard with a magical ball that will protect me from the forces of darkness. BEWAH MAH POWAH!!!