Alan Placko As you add tests, it’s inevitable that they’ll likely overlap in some way or another. If you start repeating your assertions, take the time to refactor so that if a new feature breaks something, there are still minimal things to change.

The quote you mentioned is specifically referencing the idea that you’ll never be able to test everything. When/if you find a bug, you just write a test the replicates that scenario, fix the bug in code and then hopefully it never happens again.