Negative testing is done to show that the product does not 
fail when an unexpected input is given. In other words, the 
input values may not have been represented in the 
specification of the product. But, at the end-user level, 
there are multiple scenarios that are encountered and that 
need to taken care by the product. 
The purpose of the negative testing is to break the system.
A negative test would be a product not delivering an error 
when it should or delivering an error when it should not.