This isn't a client from hell, not at all, but I thought you all might appreciate a story about one of the good ones.
I'm a graphic designer. A few years back a New Zealand women's parliamentary lobby group rang me up. They wanted to use an old but still relevant cartoon about workplace sexism that I'd drawn for a local magazine back in the 1970s. It would be part of a PowerPoint projection behind the keynote speaker at their annual conference.
Client: How much do you want for the one-off rights to use it?
I was baffled at first. No-one had ever asked me if they could use one of my old cartoons before. I didn't want to price myself off the market, but I didn't want to sell myself cheap either, so I told them I'd have to think about it, and asked them to call me back the next day.
Then it came to me. When they called back:
Me: Pay me whatever you like, but you have to pay the exact same amount to Rape Crisis, and email me proof that you did it.
There was an amused chuckle at the other end of the line.
Easiest $350 I ever made.