Both Apple's X11 and XQuartz will prompt you with a "Are you sure you want to quit now?" message if there are active clients open (actually, I don't completely remember about X11, but I'm sure XQuartz does this) unless you intentionally disabled this function using a terminal command such as: Code: defaults write org.x.x11 no_quit_alert true If there are no clients open (ie, no connections to the X server) Cmd-Q will quit immediately. If clients are open, and the default behavior is intact, you should get a warning. So if you get a warning message, don't click "Ok" because something will get ungraciously killed.
