One mistake a lot of new business owners make is forgetting or, usually more to the point, neglecting to make a business plan or create any kind of framework for their business.
In the rush to get the business up and running to make money they just dive in and hope for the best without really knowing what is involved or what they need to do when. This recklessness can certainly set the person and the business up for failure before they even get started.
A clear and sound business plan is really for you and to your benefit to have, essentially your guide on how to conduct your business and make it successful. It defines the scale of your operation, your marketing strategy, and helps identify potential risks and how to handle them should they arise. In its simplicity it gives you a heads up on what to expect and how to manage any problem or challenge you may be faced with.
While you may have a college degree in business, finance, or whatever other area of expertise and lots of experience in your field, this certainly does not guarantee that you are cut out to run your own business, especially without a business plan. It is an important first step in determining the viability of your idea and your self-discipline to build and grow a successful business.