Don’t spill salt
Have you ever seen someone spill salt and then throw some over their shoulder? It is a common belief that spilling salt is bad luck but throwing some over your shoulder afterwards will counter the bad luck (although, not for the person standing behind you who is likely to get salt in their eyes!).
It might be hard to believe, but many years ago salt was very expensive and spilling it was considered wasteful behaviour which could bring unlucky omens. An alternative theory is that spilling salt is an invitation to let the devil in and throwing salt over your shoulder will keep the devil (and bad luck) away.