Far and away, says the IRS, the most common mistake on tax returns is bad math. Numbers are transposed, left out or totaled incorrectly. And since a total figured on one tax form or work sheet often is transferred to another form (or forms), that one simple mistake could quickly compound into a very costly miscalculation.
Even if you use tax software, you're not off the hook. Sure, the computer program will do the figuring for you, but if you put the wrong numbers in to begin with, the machine won't catch that. So double-check your entries, whether on paper or on your PC.