Control+Tab is a reserved shortcut, according to Apple's Human Interface Guidelines.
Looking into it a bit further, it appears that you may have 'full keyboard access mode' enabled, as part of OS X's Accessibility features.
OS X provides the option of full keyboard access mode, in which users can navigate through windows, dialogs, menus, toolbars, and the Dock using the keyboard alone, without a mouse or other pointing device.
Users can turn on full keyboard access in the Keyboard Shortcuts pane of Keyboard & Mouse preferences. Control-F1 is a reserved keyboard shortcut for turning full keyboard access on or off. Control-F7 toggles between keyboard access for all controls in windows and dialogs and the default state, in which only text fields and scrolling lists are accessed with the keyboard. Don’t use these combinations for any other purpose.
It looks like you may be able to re-define the accessibility shortcut, and then define your custom shortcut for your mouse. Or, you might try turning off the accessibility features (if you have them enabled), and see if the shortcut becomes available.