Being used to save changes often while editing a file with development environments and editors that has the shortcut CONTROL+s can cause you a little trouble if you edit a file with vi in a terminal and you press CONTROL+s. What you get is that vi seems to hang, but the real thing that is happening is that the terminal have stopped showing output, not a vi issue at all. To solve it you simply press CONTROL+q and you will get your output back. This faq explains it.