Ha, that gives me flashbacks! I worked in a similar environment where more than once the production site(s) (multi-country deployment, big brand) would show a blank screen because some ad script did a "document.write". Or some hastily-added external dependency would stop working and render the site unusable.

The discrepancy between the care taken about deployment strategies and these regular issues always bothered me, but eventually things became more consistent. That is, our deployment strategy became haphazard and gung-ho too!