In bug 47938 <http://trac.webkit.org/changeset/70097>, we added support for tracking the unreachable URL in the UI process in WebFrameProxy. However, as soon as you start loading a new URL, any previous unreachable URL is lost. If you cancel loading the new page, the previous page is still shown, but it no longer has an unreachable URL.
<rdar://problem/9049135>
Created attachment 92121 [details] Patch
Committed r85655: <http://trac.webkit.org/changeset/85655>