Did you ever work with vi/nano (whatever)  and wanted to save a file?
If you are not that hardcore a linux person who does everything on a terminal and also works with Windows, you know that it is always a good idea to press CTRL+S once in a while to save your progress.

I press this shortcut automatically and it even happens to me while working on a putty session, which results in a “freezed” terminal session.
The reason for this behavior is that ctrl+s sends “XOFF” and putty stopps displaying any output, but still accepts keystrokes.

But its also easy to disable XOFF again – just press CTRL+Q and putty will continue to show your output on the screen. 🙂