Thinking that you can do it all and accomplish your goals without help.
Just because you might be starting a business alone, doesn’t mean that you won’t need help.  Success depends on developing and using a network of colleagues, friends, mentors and professionals that can provide advice, assistance and direction in tough times. Two professional fields that I always recommend to friends or clients who are starting up a new business are lawyers and accountants. With the vast margin or error in the legal and bookkeeping arenas of business startup, they more than pay for themselves in preventative maintenance.
Thinking for one second that you don’t need a business plan.
It's a true fact that there is a direct correlation between planning and success. By failing to plan, you are planning to fail. Some foolish entrepreneurs think that they can coast through the planning process and make adjustments to their plans as they go along. Others have the foolish misconception that a business plan would limit their creativity or spontaneity. Others still feel that their business isn't large enough or complex enough to warrant a plan.  Here’s the deal: every business can benefit from a business plan, no matter what size it is.  The process of developing a business plan organizes your strategy and helps you chart your priorities.
Assuming that you’ll make it big quickly or easily.
I’ll be honest with you; when you’re starting a business, there’s good news and bad news. The good news is that by starting a business, you’ll get to live out your dreams; that bad news is that no matter what you do, it’s never as easy as you think it will be.  There are a lot of assumptions tied up with the practice of starting a business. The reality is that success takes very long hours, strategic planning, and a die-hard commitment to the legwork involved. While the end-result is wonderful, once the work is done, the work is never as easy as you think it will be.

Not conducting a market test or doing research.
Just because you have a great idea doesn't mean you have a business.  The most common question among would-be entrepreneurs after conceptualizing a new idea is, now what? Just because you’ve thought of a great concept or idea doesn’t mean that the ‘Great Idea Fairy’ is going to visit you in the night and give you a million dollars.  Patenting, trademarking or copyrighting your great idea is a good start--right before you start writing your business plan.