I just got this error. My X server froze, though the mouse was still active.
When I switched away to a different VT and then switched back, the screens were
black.

Here are some more reports of the same problem:
https://bbs.archlinux.org/viewtopic.php?id=179906

My environment is Fedora 21 with the latest packages:

# uname -a
Linux edison 4.1.6-100.fc21.x86_64 #1 SMP Mon Aug 17 22:20:37 UTC 2015 x86_64
x86_64 x86_64 GNU/Linux

Mesa 10.4.7

I will attach the Xorg log and kernel output from journalctl.