Normally they really shouldn't, but if you have a problem writing test cases (which probably means you have a problem programming to begin with) then you may not be testing with enough variation in your input to detect corner cases, tests that should come up false etc.

An example:

  int fun(int a, int b) {
    if (a >= b) {
      return 1;
    }
    return 0;
  }

  now you could test this by saying:

  fun(4,3) should give '1' as the output

  and 

  fun(3,4) should give '0' as the output

But you'd miss to test for equality, with

  fun(3,3) which should *also* give 1 as output

So you'd need an extra test for the '=' part of the condition. Replacing '>=' with '>' will still pass the first two tests, so if you forgot about the third you will not even realize something is wrong until your code starts breaking in mysterious ways.

Unit tests have to be just as good as the code you wrote.