How a major bug in the October 2018 Update slipped past Microsoft

Last week, Microsoft began the relaunch of the Windows 10 October 2018 Update after pulling it more than a month ago due to a file deletion bug that somehow crept into the shipping build. While Microsoft has since gone into extensive detail as to how it's making sure something like this doesn't happen again, it's still unclear how such an issue made its way into the final release. So I did some digging.

Short version: Microsoft conflated two different bugs.