An excerpt from 'What do I Test?', a book on Rails testing:
Adding a new feature to an untested application is terrifying. It's like watching a bull walk into your china shop made out of playing cards. The only thing you can do is follow him around and monitor every single plate and dish he picks up. If you were sure that your walls wouldn't fall down, and that the bull was completely relaxed, you could get back to helping all the other anthropomorphic animals that have come into your china shop ready to spend their hard-earned animal money.