"shift +" while rendering in ffmpeg, how to undo?
Don't worry, all you did is increase verbosity. The Shift had no effect, but the + increased it. Press - to cycle back to a lower verbosity level.

Press ? to see a list of all available commands.

If you want to not worry about accidental keypresses, you can add -nostdin to the command. Don't do this if one of your inputs is a pipe.