Source : Free On-Line Dictionary of Computing

mumble mode
     
         The mode a program, piece of hardware, or other
        system is said to be in when it is still running and perhaps
        reacting to input and/or occasionally producing output
        (especially if it shouldn't), but in a way that appears wildly
        inappropriate to the task it is supposed to perform.
     
        Compare "{off the trolley}" and "{deep space}".
     
        (1997-03-27)