Rodzik's personal rule: never ship fake implementations alongside production code, even if it means duplicating them across test files. Duplication is cheap because fakes rarely change, and mixing test-only code with production code invites accidents. Alternatives for sharing: test helpers or factories. Separately, an audience member notes a successful technique of running the same behavior suite against both the real and fake implementations to keep them aligned, and having the team that owns the production class also own the fake.