For any somewhat complex architecture, even with great tests and canaries there will be unexpected issues.

I've yet to see the place that had the level of testing, and infallability of the CI process, to where I wanted to deploy on a Friday. Yeah, it may be a low chance that anything will go wrong, and an equally low chance we'll be unable to easily rollback, but so few things can't wait until Monday that I want to roll those dice.

I have 100% code coverage in my unit tests + behavioral tests (with cucumber).

And yet, problems still happen.

Programming is the art of adding bugs to an empty text file.