
As the system comes up, the component builders will from time to time appear,
bearing hot new versions of their pieces -- faster, smaller, more complete,
or putatively less buggy.  The replacement of a working component by a new
version requires the same systematic testing procedure that adding a new
component does, although it should require less time, for more complete and
efficient test cases will usually be available.
- Frederick Brooks Jr., "The Mythical Man Month" 
