I was summarizing the parent. Although, a few other comments here better put my point, that TDD and unit testing in general, freeze your interfaces. Because interfaces define the functionality of modules, this forces your choice of modularity upfront.

This is fine, if your already have a fair idea of how to architect your problem. But if you don't, it makes it harder to refactor your interfaces (in the same way that clients depending on your interfaces does). Of course, you can rewrite the tests, since you have control over them; it's just harder. And you don't have the TDD raison d'etre reassurance of working tests for this refactoring, of interfaces. Higher level function tests, yes; unit tests, no.