100% Calculation Guarantee
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Turbo Tax is a self prepared return. It has no way of knowing or checking that you entered something wrong or in the wrong place or entered something twice It doesn't check against any outside sources. The IRS has copies of all the W2s and 1099s your employer and banks/brokers, etc. sent in to match against. Turbo Tax only checks the calculations of what you entered. It is your responsibility to check it over before you file. They only Guarantee that the calculations are right based on the entries you made.
