Comment 1 for bug 756546

It's probably an issue with gnome-session or gnome3-session. Try this in terminal and let me know what happens:
sudo apt-get update
sudo apt-get dist-upgrade
sudo apt-get upgrade
sudo apt-get install gnome-session
sudo apt-get install gnome3-session
sudo apt-get install gnome-shell
sudo apt-get install network-manager-gnome

Otherwise, this is an upstream issue. The gnome-session, gnome3-session, and gnome-shell packages currently used belong to the GNOME3 PPA, and they need to be updated to the latest versions from GNOME's git repo.