Coupling Smells.

Production code contains test-mode flags, hooks, or branches that make the shipped code behave differently under test, so the real path ships untested.

Production classes carry members that exist only for tests — state getters, reset() methods, members widened and tagged @VisibleForTesting — polluting the real API and inviting tests that verify internals instead of behavior.

Production code whose design (tight coupling, hidden dependencies, global state, non-deterministic IO, or async-only interfaces) forces tests into awkward contortions, or makes a unit impossible to exercise in isolation at all.