There's a good post about problem clients from a consultant's point of view on Eric Wise's blog. The nub of the matter boils down to projects, clients and their expectations.
The learning point is this: once the client has started to screw around, they'll continue to see what they can get away with. And for small businesses, cash flow is king; a single client paying late can be the difference between a success and a failure.
Highly recommended reading for budding consultants.