Interesting question, since one would have to ask the question about where these superstitions came from. Walking under a ladder is a no brainer as it can be a safety hazard. Spilling salt I believe is because salt back in the day was hard to get depending on where you came from. Pennies I would assume is because when they used to have buying power, you could find one and either spend it on some candy or save up to get something really nice. Even though pennies don’t have buying power anymore, I still save them in a coin jar and deposit them in my savings account when I fill it up. Black cats were associated with witches since the Middle Ages, so the stigma still lingers.