Walking under a ladder - A leaning ladder forms a triangle with the wall and ground. Triangles represent the Holy Trinity, and violating the Trinity by breaking it (walking through it) would put you in league with the devil himself.
Spilling Salt - Salt used to be an expensive commodity used mainly for medicinal purposes. For this reason, spillage was to be avoided at all costs. The idea that it is unlucky to do so probably stems from the belief that Judas spilt salt during the last supper. Throwing spilt salt over the left shoulder is linked to its medicinal use. If it could not be administered, the next best thing was to throw it into the eye of the evil spirits that brought sickness upon us. These spirits were thought to lurk behind your shoulder, waiting for an opportunity to strike.
Breaking a mirror - Means 7 years of bad luck, unless you take the pieces outside & bury them in moonlight. Also, an undisturbed mirror in a house suddenly fall & smashes then it means that there will soon be a death.