I think I got some similar bug on Kubuntu 12.04. It happens randomly. I found this in /var/kdm.log
Backtrace: 0: /usr/bin/X (xorg_backtrace+0x37) [0xb7749627] 1: /usr/bin/X (0xb75c1000+0x18c3aa) [0xb774d3aa] 2: (vdso) (__kernel_rt_sigreturn+0x0) [0xb759e40c] 3: /usr/lib/dri/fglrx_dri.so (0xb19e0000+0xae36c5) [0xb24c36c5] Segmentation fault at address (nil) Caught signal 11 (Segmentation fault). Server aborting Please consult the The X.Org Foundation support at http://wiki.x.org for help. Please also check the log file at "/var/log/Xorg.0.log" for additional information. ddxSigGiveUp: Closing log Server terminated with error (1). Closing log file. X.Org X Server 1.11.3 Release Date: 2011-12-16 X Protocol Version 11, Revision 0 Build Operating System: Linux 2.6.42-26-generic i686 Ubuntu There's also an entry in /var/syslog: Aug 22 17:42:07 x86 kdm[1153]: X server for display :0 terminated unexpectedly -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/740785 Title: Caught signal 11 (Segmentation fault). Server aborting To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/740785/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs