On a good year, American taxpayers make between 3 and 5 million mathematical errors on their federal tax returns. On bad years — such as 2001 and 2008, when stimulus credits messed with our math — those errors can skyrocket to more than 16 million [source: Rampell]. The good news is that a math mistake, even a big one, is not necessarily going to get you on the IRS's "naughty" list. In fact, the IRS computers are so skilled at correcting miscalculations that the agency doesn't require you to file an amended return [source: IRS].
Other number blunders could cost you, though. Accidentally claiming $9,100 in charitable contributions when you only made $1,900 is a matter you'll probably need to discuss with your friendly neighborhood IRS agent.
If you are claiming dependents on your tax return, pay particularly close attention to their Social Security numbers. Double-check them. Triple-check them. Make sure that their names are spelled exactly as they're written on their Social Security cards. If the IRS computers catch a discrepancy between a dependent's name and Social Security number, that will trigger an automatic rejection of any credits or benefits tied to that dependent [source: Schnepper].