Well, if you have set a keyboard shortcut for switching input source while using GNOME 3.4. After an upgrade to GNOME 3.8, you will find that the Super+Space is the new shortcut for changing the input source (of course, you can change this in keyboard shortcuts settings). But, the old defined keyboard shortcut will also be active under the title 'Alternative switch to next source' and there will be no direct means to change or remove it.
I found the solution after digging the source code . In dconf-editor, set the following entry to its default value ( [] )
org > gnome > desktop > input-sources > xkb-options Since my old shortcut to change input source was using 'Both shift keys together', the xkb-options had the following value ['grp:shifts_toggle'] . So, in order to remove it, I set it back to []