Simply put -- over optimizing.
It's common for everyone to try and get the best deal possible in a negotiation. However, it's important to know when to stop negotiating, to say yes, and to get the deal done.
I'm currently in a negotiation which I think is falling apart because the entrepreneur is optimized the deal. We had gotten to a point where both sides of the negotiation were equally uncomfortable with the deal and had agreed to terms. Then, the request for additional terms kept coming and coming. Each additional request was small in and of itself but when added to a deal in which both sides were uncomfortable -- one small request actually was the straw that broke the deals back -- so to speak.