Mutation testing
Having 100% test coverage in your project is indeed a satisfying thing. But the higher it is, the quicker you will learn that it is never a guarantee of bullet-proof software. Countless projects with high coverage discover new bugs in parts of the code that are already covered by tests. How does that happen?
Reasons for that vary. Sometimes requirements aren't clear, and tests do not cover what they were supposed to cover. Sometimes tests include errors. In the end, tests are just code and like any other code are susceptible to bugs.
But sometimes bad tests are just empty shells—they execute some units of code and compare some results but don't actually care about really verifying software correctness. And amazingly, it is easier to fall into this trap if you really care about quality and measure the test coverage. Those empty shells are often tests written in the last stage just to achieve perfect coverage.
One of the ways to verify...