Everyone here rants about clients, and as far as I understand frustration, I understand client's side too.

For 2 years I have developed a tool for our company, my manager was responsible for outcome and was directly accountable to company's management, which made him a client for our product. Of course requirements changed many times, he pressured us much, but he is nice guy and gave us knowledge why we had to change things again. We had meetings with him, HRs, PMs and others to gain requirements for features to implement and that made me better understand client's point of view.

My point is that when you work for external companies, you only see changing requirements, pressure, deadlines, etc, but don't think that your work is just a part of process - your client is responsible for your delivery, wants to make good impression on superiors or company needs some feature ASAP. He does not have to know tech stuff, he wants outcome to be good and to be fast and cheap - that is business.

And yes - we had to tell people that X is impossible many times, had to tell Y people how things work over and over. It may seem easier when it is your own company, but note that every single employee knew that you developed that tool and you have answers for his questions.