#1. Not knowing your demographic. This happens many, many, many times. Business owners project who their target audience is. However, the projected demographic and who is actually buying can be two different audiences. So, set up some testing groups to ensure your sales will hit the right audience immediately out of the gate.
#2. Spending too much money. Yes, it takes money to make money. This doesn't mean you invest all your money on every pitch (and you will get a lot of pitches) during the first two or three years. Save your money. Only buy what you need and what has been tried and praised by other business owners.
#3. Deciding against your intuition. Your gut will always be right. Don't try to convince yourself otherwise. If something does feel, smell, taste, or look right- run like hell. Your in business because you have guts, don't be afraid to listen to them!