A small consultancy I help with had a network-wide compromise this month. The proximate cause was a single Windows vulnerability; the structural cause was platform monoculture. A walk through the trade-off.
After a year of advisories and patches, the gap between 'we patched the bug' and 'we are not vulnerable' is wider than I had appreciated. A walk through the failure modes that survive any patching regime.
Default deny is the slogan everyone agrees on. Default deny is also routinely broken, in production, by every team I have looked at, including mine. Here is the gap between the principle and the practice.