Yesterday I ran all the updates of the August patch day. Installation went smoothly. After rebooting everything worked fine. But when I shut down my notebook and switched it on a little later it came up with a blue screen with a Stop 0x50 in Win32k.sys. I could not even boot into safe mode as Windows failed to start no matter which mode chose.
So I restored from a backup, installed the updates again and... same effect. After restoring again I went through the updates and found update KB2982791 which relates to Win32k.sys and fonts. I also found this KB article (Stop error 0x50 in Win32k.sys) which says that a Stop 0x50 occurs because Win32k.sys crashes when the font cache is not correctly maintained. Quite a bizarre behaviour to crash the whole operating system because a cache seems to be invalid. A decent programmer would have just cleared the cache and continued.
Since security update KB2982791 relates to fonts, as well, I guess that MS has once again messed up an update.
From my experience with MS "support" I know that MS does not care about bug reports. So I just post this to inform people which may have been bitten by this bug. If possible, restore your OS or boot from CD and uninstall update KB2982791. Then things should work again. Oh, and after that switch off automatic updating...