Demeter violations in production code become mock chains and bloated test fixtures in test code. The Law of Demeter (“only talk to your immediate friends”) surfaces two testing problems. Object chains like a.getB().getC().doThing() force tests to mock the entire chain, and changing any link cascades failures. Fat parameters (passing a whole Customer when you need two fields) create test noise where every test constructs a complete object with irrelevant fields.
Both forms force tests to know too much about collaborator structure. The test is telling you the design needs work: push behavior toward the object that owns the data, accept only the data the function needs.