There's a bug in Gnome Shell, or possibly something beneath it (seems to apply to gdm, too) that causes a complete system lock up when I plug in an external monitor.
This does not happen when using Unity.
There aren't any logs because the system just locks up and has a blank screen. You can't get out with Ctrl+Alt+F1, nor with the special SysRq triggers.
That's the simple version of the diagnostic. The more complex bit is like:
monitor driver de result
-------- ------- ---------- -----------------------
internal intel gnome works
internal intel unity works
internal nvidia gnome works
internal nvidia unity works
external intel gnome massive crash
external intel unity Kernel buffer underrun
external nvidia gnome massive crash
external nvidia unity works
The buffer underrun I've reported against the kernel - the system was alive after, so I had the logs etc. But I'm not sure how to report the gnome problem.
edit
This got down voted because it sounded like the generic "how to report a bug". I suppose what I'm asking instead is: does anyone know of a library used by gnome and gdm that is not used by unity or lightdm, and that might be key to understanding where the fault lies? I was surprised that gdm fails, for example, and that led me to thing it's not a gnome shell issue...