Christopher Hawkins described some types of clients that are best avoided. He stumbled upon another type:
the thief. Read about in in
The 12th Abusive Client - The Thief where the client steals a site.
Reading the discussions following the blog, it seems to be a problem lots of (small?) developers have.
Even though I disliked a large part of the reaction from a guy called Fletch, he still made a point (but did not say it nicely): when the client becomes a thief, what did you do to screw up the client relationship so badly that they didn't want to pay? The point is that maintaining a good client relationship is a key factor in doing projects successfully. I assume that stealing clients are a symptom of an underlying problem: something went wrong with managing expectations from the beginning of the project.