Only if....
...while walking under the ladder on the sidewalk, your foot lands on a crack and the salt you just flung over your shoulder lands in the eyes of the person walking close behind you, causing them pain and irritation needing medical attention... :smilewinkgrin:
Personally, people are going to be superstitious, because that seems to be innate! Of course, sin is innate too, so I guess the sin is in how crazy one becomes regarding the thing they are superstitious about? And we must not forget that superstitions are a form of myths, based on groundless fallacies and delusions! With that said, it is quite possible that being superstitious to the point that it over rides one faith, then it could be classified as a sin!
Very good question!