mouse droppings n.

[MS-DOS] 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 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.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.