Comment 26 for bug 874734

The workaround it to do the following

http://askubuntu.com/questions/464221/how-do-i-disable-systems-altgrave-on-14-04

-open up a terminal run dconf-editor
-then navigate to org > gnome > desktop > wm > keybindings
-find the ['<Super>Above_Tab', '<Alt>Above_Tab']
-and set it to ['disabled']
-then it should free up that binding for you to use else where.