Similarly, it's often easy to miss some deductions altogether. Not only is the paperwork often confusing, but deductions are an unknown-unknowns situation where

Similarly, it's often easy to miss some deductions altogether. Not only is the paperwork often confusing, but deductions are an unknown-unknowns situation where you might never even see the relevant form. Several people I know ran into that this year and overpayed by enough to wipe out 5-10 years of TurboTax fees, which really calls into doubt the cost-benefit of doing them by hand. And as you said - those people were formula-happy programmers with simple W-2 income from one job. It's the absolute lowest level of complexity available, and it's still easy to get wrong.