So try this in Maven. Create a project A and don't install it. Create two subprojects, B and C, that have A as their parent. Make B depend on C. Install C. Try to build B. You won't be able to as Maven claims it can't download A. Huh? Why did C build then?
The cause is that Maven can resolve a project's parent on the local drive using a relative path. But if a project has a dependency, it will only look up that dependency's parent through the repositories, even if that parent is available via a relative path.
The solution, install A into your local repository. If another developer gets B out of your SCM and tries to build they will fail with the same problem. So I've now learned. Deploy more snapshot builds of parent POMs to keep my fellow developers happy.