Someone on my team has a parent project with several children. Let's call them Parent, Child1, and Child 2.
Parent had a number of members before any child projects were created. Parent has an edit and join policy of "Project Members Only", meaning that someone must add you to the project.
Then, someone started creating child projects. This means that Parent no longer has its own members, members are only able to be added to leaf node projects.
This has an interesting side effect. Anyone that was a member of Parent *before* Child was created is able to edit Parent. Anyone who is only a member of Child (because they cannot be directly added to Parent), is unable to edit Parent, create new Subprojects, etc.
Here are some screenshots demonstrating this weirdness:
Supertest is a member of the "PermissionsTest" parent project, via his membership of the "child permissions project" subproject.
Supertest gets this error when trying to edit "PermissionsTest"