Unit tests should test all usual and also all special cases of the functionality. If there is a special case of code that is not covered by the unit test, the code is in danger. In case of a sort implementation, the general case is sorting, say, five elements. The special cases are much more numerous usually. How does our code behave if there is only one element or if there are no elements? What if there are two? What if the elements are in reverse order? What if they are already sorted?
Usually, the special cases are not defined in the specification. The programmer has to think about it before coding, and some special cases are discovered during coding. The hard thing is that you just cannot tell if you covered ...
