Tales about bad luck

In the morning while I was studying I heard my mother say: be careful with this mirror !
Handle it with attention ! because if it falls down we have to expect seven years of bad luck!!
These words reminded me of the unusual platitudes and traditions that we have about bad luck.
For instance we are also afraid of walking under a ladder or a black cat which crosses the street while we are driving our car.
I think that all of them come from old and scary tales.
For example concern about the black cat's story I suppose I know its origin.

In the Middle Ages, people used coaches pulled by horses. At night the horses weren't able to see the black cats but could feel them pass near their feet so they got scared and started to make trouble as consequence the people seated in the coach got frightened.
I don't know if these stories are widely spread only in Naples or in the other Italian cities too, but I'm quite sure: the bad luck is everywhere :)
Do you know similar stories?

Thanks to Zsofia for correcting me