Some experts think that this belief might have started with the Last Supper in the Bible, where there were 13 people at the table. Judas, the apostle who betrayed Jesus, was the 13th member of the party to arrive. Also, Jesus was crucified on a Friday (which may have helped contribute to Friday the 13th being such a feared day).
Knock on wood
Ancient people couldn’t understand why some trees stayed green all year and others lost their leaves. They thought some trees must have supernatural powers. For that reason, they would knock on trees to get their attention.
Don’t spill salt
At one time, salt was very valuable. To spill any was believed to bring bad luck. People also thought that evil spirits lived in the left side of the body. If a person spilled salt, he or she would try to please the evil spirits by throwing salt in their direction.
Cross your fingers
This custom began as a sort of shortcut for people making the sign of the Christian cross. People believed the cross protected them from evil or bad luck.
In ancient times, people believed that their reflection in the water or in glass was really their “other self.” They thought that if you disturbed this image, you would bring bad luck. Ancient Romans believed that life is renewed every seven years. This is where we got the idea of seven years bad luck if we break a mirror, disturbing our reflection.
Walking under a ladder
This belief might be traced to the fact that a ladder leaned against a wall forms a triangle. To many Christian people, the triangle stands for the Father, Son, and Holy Spirit. If you walked under a ladder, you would break the triangle and bring bad luck.
Source: The Orange County Register's "Mini Page"