Dropping salt is not really bad. It is a superstition whose origin is dated to the ancient times. According to the myth, in order to prevent any bad luck occurring, the salt spiller should toss some of the spilled salt over the left shoulder.
Salt was very valuable in Jesus's time. It was thought that the devil spilled the salt and that you should throw some over your shoulder into his eye so he would go away. Look close at a painting of the last supper and you will see salt spilled in front of Judas.