Tests freeze code and design. Often this is exactly what you want but sometimes it's not what you want at all. If your internal interfaces are unit tested the

Tests freeze code and design. Often this is exactly what you want but sometimes it's not what you want at all. If your internal interfaces are unit tested then you can't refactor easily because you'll break all your tests. However, if only your external interface is unit tested then those tests actively benefit refactoring the internals.