Comment 3 for bug 992391

I can add more info.

1. I can not reproduce freeze if I stop lightdm and launch just xterm

startx /usr/bin/startx

Although when I switch between LVDS1 and VGA1

xrandr --output LVDS1 --off --output VGA1 --auto
xrandr --output VGA1 --off --output LVDS1 --auto

approximately in a half of cases I get blank screen. I can restore the image by switching to a console [Ctrl+Alt+F1] and back.
I do not see any errors in dmesg or Xorg.0.log

2. I have not managed to reproduce freeze in unity-2d when only xterm is running. The problem with blank screen persists. After several switching I have noticed

[drm:drm_mode_getfb] *ERROR* invalid framebuffer id

in dmesg output. More switching leads to more such lines.
At certain moment

xrandr --output VGA1 --off --output LVDS1 --auto

failed with the following error:
xrandr: cannot find crtc for output LVDS1

Nothing unusual in xrandr output:

xrandr
Screen 0: minimum 320 x 200, current 1600 x 1200, maximum 8192 x 8192
LVDS1 connected (normal left inverted right x axis y axis)
   1280x800 60.0 +
   1024x768 60.0
   800x600 60.3 56.2
   640x480 59.9
VGA1 connected 1600x1200+0+0 (normal left inverted right x axis y axis) 408mm x 306mm
   1600x1200 59.8*+
   1280x1024 75.0 60.0
   1152x864 75.0
   1024x768 85.0 75.1 60.0
   832x624 74.6
   800x600 85.1 75.0 60.3
   640x480 75.0 60.0 59.9
   720x400 70.1

Surprisingly [Fn+F6] switched image to internal screen and xrandr started to work again.

I am afraid that some applications will be able to freeze X server even when unity-2d is running.