Never act too confident or arrogant with customers. Always be respectful to employees and remember that you are in their workplace. Respect the culture and the impact that decisions, based on your consulting, will have on employees. Misunderstandings, setbacks, delays, and other problems are problems that business owners face when dealing with a consultant.
While the lack of competence on both sides and other factors may contribute to these problems, another reason why these problems occur is that business owners and CEOs expect consultants to step in and perform tasks that are outside their scope of work. New consultants often struggle to manage customer expectations and may end up working late into the night and on weekends just to meet a deadline. It's important to set limits early on so that you can continue to be productive and not work too hard. This is strongly related to point 1; don't underestimate what you promise your customer from the start.