Salt is a very valuable substance in many cultures and rituals. For example, salt represented trust and friendship in most cultures in which salt was involved. It is also used in many religious rituals such as when a priest performs an exorcism on a person believed to be possessed by demons, or whenever the Roman Church rite made holy water that could then be used in the exorcisms. However, with all its good points in culture, there is also its negative points in superstition.
It is considered to be bad luck when you accidentally spill some salt. The reason as to why this is considered as bad luck is unknown.
One belief is that Judas Iscariot, one of Jesus Christ's 12 Apostles, accidentally spilled some salt on the table during the Last Supper. Said disciple would then betray Jesus by turning him over to the authorities to await his upcoming trial and execution. This idea was adapted into the famous The Last Supper painting by Leonardo Da Vinci in the 1490s.
A common way to fix your mistake is by taking a pinch of salt and throwing it over your left shoulder. According to legend, this was meant to confuse The Devil by throwing salt into his eyes.