Well, the above says it all, really - one of the sites I maintain makes extensive use of the navigation view to create navbars, and I just added two new pages to the site. When I went to add them to the correct place in the navigation view, the first was added, no problem, but the second just sprouted a 'no entry' sign as soon as I dragged it onto the view's screen.
I tried closing FP and re-opening it in case it had got its knickers in a twist but it was exactly the same again.
Are there limits to how many pages can be added, or something? I've never heard of any such limits, and I only have about 50-odd pages total so I can't see that it should be anywhere near them if there are!