This isn’t a story about a client from hell, but I know for a fact that it is advice that comes from dealing with them.
When I was studying 3D animation and visualization, we did customer projects as a part of our education. For each project we had a tutor, someone who worked in the industry and knew the ropes. This is legitimately one of the lessons one of our tutors gave us:
Tutor: In our company, we have what we call an “Ugly Red Blob” method. When a design has been finalized, and it’s time to send it to the client for final review, the last thing we do is we add a big, red blob somewhere in the design. The client will look at the mock-up and say: “Okay, that’s good, but take out that red blob, it doesn’t work with the rest.” The client gets a feeling that they’ve contributed to the design in a significant way, and we can get on with our work, without the client trying to micro-manage every font and color.