Errors in case role assignment are silent

I'm not sure how to give repro steps that don't involve database hacking. In my particular case, I was getting a duplicate relationship error because the relationship already existed between the case client and the contact in question, but it had (somehow) been flagged as inactive.

To reproduce it, I suppose you could create a case relationship then disable it via the database. If you then try to create the relationship again, the page reloads in much the same way it does during a successful save, and the user gets no indication that there is a problem.