It is a well-known superstition that breaking a mirror brings seven years of bad luck. Of course there is a much more reasonable explanation for warning people to avoid breaking mirrors.
One reason is that mirrors are (especially in the past) made of glass. Broken mirrors and glass are really dangerous. Not only can they cause nasty cuts, but the little shards can go unnoticed for some time, even after cleaning up, and can get stepped on and enter the blood stream and kill (or at least cause other health problems).
Another reason that people were warned to avoid breaking mirrors is because mirrors used to be made of silver spread thinly on the back of the glass (silver is rarely used these days). As such, they were expensive and so breaking them was undesirable.