https://www.theregister.co.uk/2018/10/10/microsoft_windows_deletion_bug/

The world now knows why last week's Microsoft Windows 10 upgrade deleted unlucky users' files: the software treated the default user directory as ripe for destruction, because it thought the files were elsewhere. The upgrade has since been pulled.

Last week, some unfortunate punters who hit the “download” button on the Windows 10 October 2018 Update received the gift of extra disk space, because they lost their \Documents directories.