If there's one thing I've learned over many years writing lots and lots of automated tests for everything is that no matter how far you go with your tests, barring formal proofs or equivalent, the tests cannot save you from, at some point, breaking production badly. One small, tiny thing that slipped through your tests and boom - huge fuck up (even small things can easily cause your whole new feature to fail miserably on the big launch).
What big launch? :) Is useful to feature flag new functionality so that you can release gradually and spot the bugs - often via error monitoring and user feedback - before they affect everyone. Yes, realize it can happen but there are techniques to minimize the chance of it happening.
