When all tests are done and you think you’ve done enough to justify a release, think again. If there is still time, sleep a night over it.
The extra time will allow you to defocus, to be creative, and to look for other ways. If you step back from your tests, you usually have a better eye for the bigger picture. It results in new “what happens if…”-questions, new test approaches, and new test-techniques.
The extra time needed depends on the project, the person, and the System Under Test. Sometimes a cup of coffee or a go to the toilet is sufficient. In other cases a night or a weekend gives better results.
In my experience new tests will pop-up and some of them even reveal some yet undiscovered bugs. It is often well worth the extra time.