I came across this in a game this morning. A chain of mountains blocked all access to a section of the map. Here is a screenshot:

It's hard to tell from the pic, but the mountains continue down a little ways until they hit water again. Basicly there is a penisula blocked off entirely by mountains. Also, if you look closely, you can see a red border at the top next to my white border, so apparently some unlucky AI started behind those mountains and can't really do anything.

I don't think this has been reported yet, but there are way too many bug posts to scroll through, and a quick search yielded nothing.