Comment 4 for bug 929655

There are errors about invalid framebuffers:

Xorg.0.log:
(EE) intel(0): Couldn't create pixmap for fbcon

dmesg:
[11678.477663] init: tty1 main process ended, respawning
[11680.331193] [drm:drm_mode_getfb] *ERROR* invalid framebuffer id

However these are not likely to be the cause of your issue.

Given that killing compiz restores the system to functionality, this tends to suggest a bug in compiz, unity, or one of the compiz plugins.

Reproduce the bug and capture your ~/.xsession-errors while the system is frozen, and attach that here. Hopefully the tail end of that will contain a pertinent error message.

You may also want to review what compiz plugins you have enabled or any compiz settings you may have made. Or, try logging into a guest session and see if the error reproduces there.