The error below is reported after an X session fails. The session has shown to fail at essentially random times, some example circumstances: while closing a pdf, dragging a file into the GIMP, while the system is idle with a locked screen (afk). The screen will flicker black and I am greeted with a new graphical login screen. In a console session I see that the user is no longer logged in.

~$ cat /var/log/Xorg.0.log.old

...

Backtrace:
0: /usr/bin/X(xf86SigHandler+0x84) [0x80c43e4]
1: [0xffffe420]
2: /usr/bin/X(ProcFreePixmap+0x83) [0x8081323]
3: /usr/bin/X(Dispatch+0x19b) [0x8086ccb]
4: /usr/bin/X(main+0x489) [0x806e6b9]
5: /lib/libc.so.6(__libc_start_main+0xe0) [0xb7dc5450]
6: /usr/bin/X(FontFileCompleteXLFD+0xa5) [0x806d9f1]

Fatal server error:
Caught signal 11.  Server aborting