When you build your test plans, it’s quite important to keep in mind that testing for positives is always easier to maintain. Furthermore, by testing for positives, you are effectively testing and documenting your actual requirements.
Tests should follow what I call the golden path. The golden path is the most direct path to reach your goal without taking any of the permutations into account. This means that If I’m testing a login screen, I will test each of its features separately. Just to be clear, a failed login which provides the user with the proper feedback is a feature. Error messages are part of the requirements and should have specific tests.
Don’t test for unknowns. Testing for something that is undefined or unpredictable can greatly complicate your test plans and can lead to false positives. Testing for negations can pretty much result the same way.