• Sometimes it's the scope of the unit tests. There are some situations with complex UI, business rules and databases where it's difficult to build automated coverage beyond a certain percentage. Or the unit tests approach the complexity of the code under test and require unit tests of their own... It's turtles all the way down.

    A recent project I did, spent 85% of the time creating and maintain unit tests, only to have a unique corner case not covered by tests come in from the field and require refactoring of the code and tests.