Comment 5 for bug 1344752

Ok, searching"xfce ctrl space" or "ubuntu ctrl space" one case that others have found this to be an issue is with IBus. See the links for description and workaround.
http://askubuntu.com/q/243639
http://peter-butkovic.blogspot.com/2013/05/keyboard-shortcut-ctrlspace-caught-in.html