1. Missing domain.
Even if it doesn't seem necessary - always enter a domain.
2. Sandbox Mode
Page Admins can see Apps even if they are in sandbox mode. If those page admins aren't app admins or testers though, they might just get this facebook error. I spent a lot of time looking for this problem until I realized one of my clients was only a page admin and tried to test the app prematurely.The solution to this is, of course: make page admins app admins/testers.