Source : Free On-Line Dictionary of Computing

mouse droppings
     
        1.  {Pixels} (usually
        single) that are not properly restored when the {mouse
        pointer} moves away from a particular location on the screen,
        producing the appearance that the mouse pointer has left
        droppings behind.  The major causes for this problem are
        {MS-DOS} programs that write to the screen memory
        corresponding to the mouse pointer's current location without
        hiding the mouse pointer first, and mouse drivers that do not
        quite support the {graphics mode} in use.
     
        2.  The client address recorded in a
        {World-Wide Web} server's log whenever a client connects to a
        site.
     
        Users may be unaware that their activity is being logged in
        this way but the potential for misuse of the information is
        limited.
     
        [March 1996 Macworld, p260, Viewpoint article by Larry Irving].
     
        (1994-12-05)