DailyTech posted a story that a recent released security patch caused the infamous blue screen of death (BSOD) on some Windows 8.1 PCs

The error (known more technically as Stop 0x50) reportedly occurred for a small number of users with the following updates:

KB2970228 - Non-security optional patch for new Russian Ruble symbol
KB2975719 - Patch Tuesday update set (includes KB2970228)
KB2976897 - Kernel Patch
MS 14-045/KB2982791 - Kernel Patch

The '897 and '791 patches modify Win32k.sys/Gdi32.dll, so copying those from a working machine of the same version of Windows will negate the effects of those patches (according to Redditors). Patch '228 creates a troubled temporary file called fntcache.dat -- which can simply be deleted to apparently fix that part of the problem.

  Microsoft: Roughly 25,000 Win8.x Users See Blue Screens After Botched Patch