Comment 3 for bug 1385572

I don't know how much this comment would contribute to this bug but I can at least confirm that the behavior described here is a possible reason why this is going on, as judged by my comment to bug 1236749 here: https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/1236749/comments/77 Specifically, for me the bug only occurred if I rebooted cleanly, but not if I forced the machine off.