Doing too many tasks on your own instead of hiring or outsourcing.
Many small business owners are eager to jump into their business and do everything from the start. That's a great attitude to have but I think you'll quickly find that you will be overcome with what Michael Gerber calls working in the business rather than on the business. There is a very important distinction to be made here.
Working in the business means paying bills, handling payroll, finding the best deals on internet and phone service and making sure the fax machine you have is still the best fit for you.
Working on the business means doing the sort of things that actually pay the bills (including payroll) and make you happy. This is the reason you started your business, is it not?
Grab a notepad right now and list out the things that make up your business day. Put down everything, even if you're not working in your business full-time yet. You have to design business cards, handle website maintenance and, oh yeah, actually talk with your customers.
Begin investigating what it would take to outsource some of the tasks of your business in order to both free up your time and increase your business revenues.