X seems to randomly crash several times a day and log you out on certain Nvidia cards (e.g. Quadro FX 3800, 9800GT) using proprietary drivers newer than 290.10. Reverting to X.Org X Server 1.7.7, seems to resolve the problem. I've encountered this both on Ubuntu 12.04 and Fedora 17. For more details see this bug report over at Launchepad: https://bugs.launchpad.net/ubuntu/+source/nvidia-graphics-drivers/+bug/973096 I was unable to find anything about it it in the Xorg bug tracker, and the issue seems to be either with Xorg or Nvidia, or the communication between the two.
Fixed downstream as per https://bugs.launchpad.net/ubuntu/+source/nvidia-graphics-drivers/+bug/973096/comments/311 .
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