This bug affects me as well. Running 64 bit Ubuntu 12.04 with nvidia graphics (stable version). Happens when playing flash videos in Firefox 12.0. Switching to Ubuntu 2D on login screen helps.
Dmesg says: Xorg[8478] general protection ip:7f5f27e466a9 sp:7fffdda31ff0 error:0 in nvidia_drv.so[7f5f27de5000+6e0000] /var/log/syslog says: May 11 22:55:47 kernel: [22859.617181] Xorg[8478] general protection ip:7f5f27e466a9 sp:7fffdda31ff0 error:0 in nvidia_drv.so[7f5f27de5000+6e0000] May 11 22:55:47 gnome-session[8647]: Gdk-WARNING: gnome-session: Fatal IO error 11 (Zasoby chwilowo niedostępne) on X server :1.#012 May 11 22:55:48 acpid: client 8478[0:0] has disconnected May 11 22:55:48 acpid: client 8478[0:0] has disconnected May 11 22:55:48 acpid: client connected from 12626[0:0] May 11 22:55:48 acpid: 1 client rule loaded May 11 22:55:49 acpid: client connected from 12626[0:0] May 11 22:55:49 acpid: 1 client rule loaded -- You received this bug notification because you are a member of Ubuntu-X, which is subscribed to xorg in Ubuntu. https://bugs.launchpad.net/bugs/980519 Title: Random log off in Ubuntu 12.04 LTS To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/nvidia-graphics-drivers/+bug/980519/+subscriptions _______________________________________________ Mailing list: https://launchpad.net/~ubuntu-x-swat Post to : ubuntu-x-swat@lists.launchpad.net Unsubscribe : https://launchpad.net/~ubuntu-x-swat More help : https://help.launchpad.net/ListHelp