If you run any type of business or a freelancer which provides some sort of service, you will always come across some absolutely horrific clients which are just impossible to deal with. I feel like designers and developers come across the worse clients though.
The problem is that a lot of people who are looking for design work don't actually know what they want, or know what they want but won't tell you what they want, they just expect you to know. Trust me, I've had this from one client myself.
They also don't seem to understand that design work is not exactly cheap. Most want it done as cheaply as possible and think that because designers "love" what they do, they should do it for cheap or even for free! I mean like the cheek some people have.
Clients From Hell is an absolutely hilarious Tumblr blog where people have submitted their horrific experiences with clients, mostly because the clients are just clearly dumb as shit. It's a great time killer.