I have open borders stuck on with a civilization, despite having no current open borders agreement (as verified from the current deals list). I'm not entirely sure when this started happening, but I believe that it was immediately after signing a peace treaty (and possibly in the situation that I had had Open Borders before the war started). This lasted for the rest of the game (which was significantly more than 30 turns). Save attached from when I first noticed this. I have two-way open borders with Siam, but should have no open borders.