Coverage is useful because it shows where tests did not go. It becomes misleading when teams treat it as proof that the system is safe.
You can drive line coverage very high and still miss the behaviors that actually hurt the business:
- authorization mistakes
- concurrency bugs
- integration breakage
- domain edge cases
Why Coverage Misleads
Coverage answers a narrow question: did this code execute during a test run?
It does not answer:
- was the right assertion made?
- was the dangerous branch exercised under realistic conditions?
- did the external system behave correctly?
A payment service can hit 95% coverage and still duplicate charges if nobody tested retry behavior with the right domain contract.
Better Testing Lens
Good test strategy starts with risk, not with percentages:
- unit tests for core decision logic
- integration tests for external boundaries
- scenario tests for business rules and failure modes
- a smaller number of end-to-end tests for critical journeys
Coverage is still useful as a map. It helps reveal blind spots. It just is not the quality verdict by itself.
Better Rule
Use coverage to ask, "What code are we not exercising?" Do not use it to claim, "The system is safe."
Further Reading