Earlier this week, Microsoft started distributing the Windows 10 October 2018 Update, version 1809, to Windows users who manually checked for updates. The company has now halted that rollout after many reports that installing the update is causing serious data loss: specifically, deleting the Documents, and perhaps Pictures, folders. Microsoft is also advising anyone who has downloaded the update but not yet installed it to not install it at all.

The exact circumstances causing data loss aren't clear; the handful of reports on Microsoft's forums and Reddit don't have any obvious commonalities, and people report seeing only one affected system among many when upgraded. There will need to be some amount of investigation before a fix can be developed.

This will be too late for anyone that's suffered data loss; although file recovery/undelete tools might be able to salvage the deleted files, the only reliable way of recovering them is to restore from a backup.

A data-loss bug is bad. Data-loss bugs are the worst kind of bug that Microsoft could ship; for rarely backed-up home users, at least, they're worse even than a security flaw—who needs hackers and malware to destroy your data when the operating system does it for you? This bug is sure to raise new doubts about Microsoft's testing, pace of delivering updates, and dependence on the Insider Program to find and report such problems.

Sampling of Feedback Hub reports of data deletion from as early as 3 months ago pic.twitter.com/xlfsuEbh1d — Rafael Rivera (@WithinRafael) October 6, 2018

Making this worse is that the bug does appear to have been reported. Numerous reports in Feedback Hub, Microsoft's bug-reporting tool for Windows 10, complain of data deletion after installing preview releases. None of the bug reports appears to have many upvotes, and the reports generally lack in detail. So just as with the more recent reports, they make it hard to pin down the root cause. But it's obvious that, at the very least, something was going wrong and that it was important enough that it should have been investigated and addressed.

Compounding this issue is that Microsoft's rollout of version 1809 was already unusual. For reasons unknown, Microsoft didn't release this update to the Release Preview ring, so the most realistic installation scenario—someone going from version 1803 to 1809—didn't receive much testing anyway. And all this is against the longer-term concern that Microsoft laid off many dedicated testers without really replacing the testing that those testers were doing.