Projects are complicated. Lots of agreements and changes are constantly made, these should be managed properly.
Document all changes, variations and agreements on your projects. You should put everything in writing.
Keep good control of client communication. Ensure all important communications are between the project manager and the client project manager, especially for cost, schedule and scope decisions.
When you are making important decisions or preparing designs, get a second opinion on them before acting, because you may be coming from a biased or primed attitude. (Primed is …