What's the "Spell Hint" is on Ubuntu 12.04

I met the same problem with Ubuntu 14.04. In my case, it seems to be related with my keyboard?/language? input method Fcitx (for Japanese input) rather than Ubuntu. So if you don't use Fcitx, my advice might not be useful. Or it might be a problem of Japanese keyboard/input.

You can find "Input Method Configuration" window by clicking a top-right keyboard icon. In "Input Method tab" of the window, there will be list of your keyboard and input method. Double-click your keyboard in the list, "Toggle the word hint" field uses Ctrl+Alt+H. Click Ctrl+Alt+H then hit "Backspace" key, the value will turn "Empty", now you can use Ctrl+Alt+H as usual.