Friday the thirteenth: The Scandinavians believed that the number thirteen was unlucky due to the twelve mythological demigods being joined by a thirteenth, an evil one, who brought misfortune upon humans. It was also said that Christ was crucified on Friday and the number of guests at the party of the Last Supper was thirteen, with the thirteenth guest being Judas, the traitor.
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.
Black cats: Although sometimes thought to be a symbol of good luck (in ancient Egypt, the Goddess Bast was a black, female cat), during the Middle ages cats were thought to be demons in disguise and should thus be destroyed.
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.
Other superstitions associated with bad luck and misfortune include: A bat flying into the house. An owl hooting 3 times. 3 butterflies together. Looking at the new moon over your left shoulder. A 5-leaved clover. Breaking a glass whilst proposing a toast. Putting a shirt on inside out. Red and white flowers together. Hearing a rooster crow at night. Cutting your nails on a Friday.
Putting a hat on a bed. Getting out of bed left foot first. Violets blooming out of season. A picture falling. Breaking a mirror. Singing before breakfast. Opening an umbrella indoors. Giving away a wedding present. Stepping on cracks in the sidewalk. An itch inside your nose. Crossed knives. Seeing an owl during daylight. Spilling pepper. Killing a sparrow. Black birds.
Dropping a wedding ring. Breaking a plate, especially if it has not already been cracked. Mending a garment whilst wearing it. Signing contracts in the months of April, July or November. Spilling ink. Breaking a bottle.
Fastening a button into the wrong buttonhole. A candle falling over. Stepping on board a ship with your left foot. Tripping over when you leave your house. [1]