Founded in 1993 by brothers Tom and David Gardner, The Motley Fool helps millions of people attain financial freedom through our website, podcasts, books, newspaper column, radio show, and premium investing services. No one likes to pay taxes, and you definitely don’t want to pay more than you actually owe. Unfortunately, the tax code is really complicated in the United States, so it’s far too easy to make a costly mistake. Simple errors could cost hundreds or even thousands of dollars if you don’t get things right the first time when dealing with the IRS. To help you avoid errors that could come back to haunt you, here are four big mistakes to be aware of. The U.S. has a pay-as-you-earn tax system, so most people can’t just wait until the end of the year to pay all that you owe to the IRS. For most people, this isn’t a big deal because employers withhold an appropriate sum based on information you provide on your W-4 form when you’re hired. As long as you provide accurate information, employers will send the correct amount — and often a little extra — to the IRS.  But if you earn income outside of the traditional employment relati...