When no connection is made with any other machine, but you "accidently" hit the "Render Still" button, the whole window for render settings (so the one on the right side of the screen) moves suddenly fully up and disappears from the screen. The "render engine" field on top of the screen is empty (emptied?) and system freezes. When the slider for the render settings window is moved up, then all normal windows are gone:
CR should give a warning or: "No machine connected. Please connect or use default render buttons"