If you have a subdirs project open and you add a new sub-project (a unit test project for example) then the new project does not appear in the Build/Run target selector so you cannot run it or debug it. The only way to make it appear is to close the project, delete the .pro.user file and reopen it. This means that any custom build steps, etc. are lost. This is particularly frustrating for unit test projects where each class has a separate test project.