Test coverage is a metric, and any metric can be gamed. Test coverage can be gamed very easily: Ensure that for every piece of code, there is a test that runs it. However:

• Don’t assert anything
• Only execute one branch (unless branch coverage is enabled)
• Catch and ignore all exceptions

Now you have 100% test coverage and a useless test suite.

See also

• Week­notes 2023 W19: Stub, where I talk about how stubbing can decrease the value of test suites.
• Don’t set test coverage delta requirements