"Adding manpower to a late software project makes it later" - a result of the fact that the expected advantage from splitting work among N programmers is O(N) (that is, proportional to N), but the complexity and communications cost associated with coordinating and then merging their work is O(N^2) (that is, proportional to the square of N).
The quote is from Fred Brooks, a manager of IBM's OS/360 project and author of "The Mythical Man-Month".
The myth in question has been most tersely expressed as "Programmer time is fungible" and Brooks established conclusively that it is not. Hackers have never forgotten his advice; too often, management still does.
See also creationism, second-system effect, optimism.