I am not changing any subject. These are glue logic scripts connecting massive pieces of infra together, spanning what is likely several teams and orgs over the course of many years. It is impossible to blurt something out like "well, source code testing" for something like this, when the source code inputs are not possibly testable outside the scale of the larger system. They're often completely unknowable as well.

With all due respect, it sounds like you have not worked on these types of systems, but out of curiosity - what type of test do you think would have prevented this?