Not defining your target market.
"You mean I can't work with just anyone?"
I get this question a lot. Why would I intentionally shut certain people out of my business if my services are for everyone?
Most people in North America are in debt and manage their money poorly. I can help almost anyone improve their relationship with their spouse and manage their money better, but if I market to everyone, essentially I reach no one.
I met a financial planner recently who targets successful women and their families. At first glance he just shut out about half of the country by not targeting men. And then he defined his market as successful women. Talk about a great niche!
But just because you designate a specific niche doesn't mean you can't work with other types of clients. My friend will work with just about anyone that comes to him. As he markets his business however, he is able to fine-tune and specify exactly the kind of clients he's looking for and go directly to the source to find them.
The more focused you are, the more bang for your buck that you get when it comes to marketing.
Auto dealers do this exact same thing. The Honda dealer would love to work on your Accord. You'll pay a lot more than at the shop down the street. There is some implied reasoning that the Honda dealer will do a better job, although some could argue that. Even though the mechanics at Honda can fix your Accord, they will also fix your Ford or GM. But you'll never see that in their marketing.