[Boreas]

Spilling salt is considered unlucky because Judas spilled the salt at the Last Supper. Throwing some of the spilled salt over your left shoulder is thought to be throwing it in the Devil's face, thereby warding the evil. You wouldn't throw it over your right shoulder because that's where your Guardian Angel stands.

But the idea of spilled salt being unlucky is even older. Salt was a valuable commodity because it kept food from spoiling. As such, in Roman times, it was often given to dinner guests as a gift symbolizing friendship. Spilling the salt was thought to put those friendships at risk.

As you can see, this earlier Roman belief is probably behind the Christian belief concerning the Last Supper. So often, the deeper you dig into a superstition, the more you have to dig. Who knows what earlier superstition (or event) lies behind the Roman one?

John