Bug#647077: gnome-shell: segmentation fault when gnome shell starts

[Alex Page]

Are you running separate X sessions, by any chance?  I had this same 
problem running gnome-shell --replace:

Window manager warning: Log level 8: gtk_style_context_get: assertion 
`priv->widget_path != NULL' failed
Segmentation fault

I could get into a session, but had no top panel, menu, etc... 
Switching to Twinview, the problem went away.

Now I can't use my 3rd monitor, though...