You must have never worked on a project with more than 10 people on it.
Indeed, and it no doubt passed all the unit tests and was marked as good to ship.
But I suspect what they were lacking was an aggressive, adversarial QA specialist with the mindset of "how would a user break this in daily use".
Isn't the point of a beta release to make sure that the OS is battle-tested with users who would use it like they do in real life?
Would have thought it might have covered most of the typical real-world use cases. Especially since it probably has one of the highest number of users/testers of any beta software programs.