I believe that 100% test coverage is not a good idea in applications. However, I find it very very valuable in libraries. I have a few personal recommendations:
Avoid white-box testing. It makes refactoring harder. We should be able to achieve 100% test coverage using only the public API of the library.
Every reported bug should be reproduced with a unit test before it is fixed. These unit tests are more important than achieving 100% coverage.
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