Step to reproduce :
Login in System and untick 'Single Row Layout' on User Assignment tab of Role window.
Log out and login with SuperUser.
Goto the Role window, and click once on User Assignment tab (it must stay in detail position).
Delete the actual record System Administrator / SuperUser.
Click on 'New Record' (button in detail toolbar). Then click on Save (master toolbar). You won't be able to save until the detail tab become the master one.
Found the issue is because the UI is not aware that detail record is ready to be saved when you are just inserting and it has all default values, if you touch any field then the save works.
Reproduced by Anton Mostovenko.