I agree with the others, the most common mistake is spending too much time planning and not enough time executing. IMHO, the second biggest mistake is not having someone on the team with a successful (let me repeat, a SUCCESSFUL track record in sales). It is common in startups that the President/CEO/Chief Bottle Washer is also the lead sales guy. If the swiss-army employee doesn't have the scars from a stint in the field, then they should start building and honing the skills to start the pipeline moving. A book I recommend to every sales person is the classic "Solution Selling by Michael Bosworth
Over estimating their idea and under estimating the competition. It is all a matter of degrees. If you think you are WAY better than the compitition - you get sloppy. Winning in business is like the 100 meter dash at the Olympics - the difference between winner and last place is .01 of a second! You win by making sure that you are incrementally better than the rest. And you are always aware of where they are and what it takes to beat them.