This happens to me on daily basis, sometimes even more often. Some magical keyboard mispress on the german keyboard, produces these characters that Xcode can't show. Editing out characters, that you don't see is pretty hard. And "Show Control Characters" doesn't show them either. So does Xcode let them entered in the first place ?
Solution ? Reveal in Finder, use emacs to edit them out. Then I ponder over the reasons, why I am not using emacs and make in the first place.