One theory of the origin of not upsetting the salt cellar has more to do with etiquette than warding off bad luck. In ancient times, salt was expensive — so precious that, in some civilizations, it was currency [source: Pettit]. Spilling salt was tantamount to throwing away money, and only the devil would cause someone to act with such foolishness. Tossing a pinch over your shoulder would blind the devil and make him think twice about trying to trick you again.
If you spill the salt, you must be the one to throw it over your shoulder, or you won't thwart the bad luck. If you're the superstitious type, or just like to play it safe, it probably won’t hurt to toss a little pinch of that sodium chloride over your left shoulder should you accidentally spill some. You never know what might be lurking behind you!