Adding [human resources] to a late software project makes it later.
This law first appeared in a classic book, The Mythical Man-Month. The central insight is that new developers require onboarding, which in turn requires time of existing developers, which in turn subtracts time from ongoing projects. Hence, a word of caution for hapless managers who attempt to speed up a project which is running off the rails: adding new resources will only exacerbate the problem.
The qualifier “late” in the statement of the law is an important one. It suggests that human resources can be added to projects which are not late. Hence, for example, the success of some open source software projects, which successfully incorporate thousands of developers over the course of their lifetimes.