Comment 4 for bug 939858

I think the problem is that you're simply running out of video memory. Each user session runs in its own Xserver. Each Xserver needs video memory for various render and display buffers. A number of factors influence video memory usage:

* number and size of attached monitors
* virtual desktop size
* driver features requiring additional video memory per monitor:
  - monitor rotation
  - tear-free desktop
  - OpenGL overlay
  - stereo 3D
* 3D OpenGL applications
* composite manager
* 2D applications

If the system has been running for a while, video memory fragmentation can also prevent successful allocation of the required buffers even if the raw amount of free memory would be sufficient.