I've observed a strange behavior of the EditPage. When $wgRawHtml = true and a user has the option previewonfirst ("Preferences > Editing > Preview > Show preview on first edit") enabled, the wiki displays the session_fail_preview_html message when the user navigates to action=edit. This is due to the logic in . I've examined the program flow and came to the conclusion that in the described scenario $this->mTokenOk can never be true, as no one ever calls $this->tokenOk( $request ). Even if the method would be called no wpEditToken would be available in the request, as it is just a initial call to action=edit.
It occurred in a current REL1_27 code base, but logic in HEAD seems not to be very different, so the issue might still be there.