I have the same problem in 14.04. As mentioned at https://bugs.launchpad.net/ubuntu/+source/unity/+bug/1301623
it is possible to reproduce the problem by dragging around windows in the workspace switcher, grabbing them by the title bar (especially, it seems, by the bubble buttons in the title bar), and I can regain control by pressing super+S, exiting the workspace switcher.
I have the same problem in 14.04. As mentioned at /bugs.launchpad .net/ubuntu/ +source/ unity/+ bug/1301623
https:/
it is possible to reproduce the problem by dragging around windows in the workspace switcher, grabbing them by the title bar (especially, it seems, by the bubble buttons in the title bar), and I can regain control by pressing super+S, exiting the workspace switcher.