As much as I like unit tests, and the whole meme of test-driven development, there is a kind of circular argument that goes something like

1. Unit tests catch all errors introduced by refactoring
2. ...except for those error it doesn't catch
3. But you add tests for these errors when you notice them
4. ...therefore (see 1) unit tests catch all errors introduced by refactoring

It reminds me of an infinite regression, a la Zeno's paradox (the one about the tortoise in a footrace).