Salt wasn’t always as common as it is today. It used to be a valuable commodity, and people often viewed the substance with mystical significance due to its use as a preservative and purifier. Soldiers in ancient Rome were sometimes even paid in salt, which was considered fine by them because they could not only add a dash to their supper, but also use it to protect themselves against evil spirits. It turns out that there are thousands of malevolent ectoplasmic entities lurking in the air around us, more specifically right behind our left shoulder.
If you spilled salt, this was not only a tremendous waste, but a sign of bad luck. The malicious spirits were going to send mischief your way very soon, though this could be prevented by throwing a pinch of salt into their faces. This would cause them to rub their eyes and mutter about the inconsiderate nature of people before floating off to find a different poor soul to bother. This still holds true to this day, so the next time you accidentally knock over the salt shaker at dinner time, be sure to blind the little buggers with a quick toss before they do irreparable harm to you.