Everyone in America knows that the number 13 is considered an unlucky number. We observe that in a lot of strange ways, considering that it is just a superstition. But you will not find the 13 on a sport uniform, there is not 13th floor of any high rise building, and Friday the 13th is widely believed to be a bad luck day.
Strangely, people in other cultures have assigned the same significance to different numbers. So this must be a human failing – giving numbers power over our behavior is just weird to me.