What does the guarantee actually cover? I was told it would be 100% accurate, if it's not will you fix the difference if the refund isn't what they listed?

[SweetieJean]

TT stands by its calculations, but has no way of knowing if (for example), you made a data entry error, or forgot to claim some income.  See:

TurboTax® Software License Agreements - Learn About TurboTax Software Agreements (intuit.com)

[DavidD66]

The accuracy guarantee says  "If you pay an IRS or state penalty or interest because of a TurboTax calculation error, we'll pay you the penalty and interest."  

If you make an error entering your information or you omit something, and the IRS adjusts your return and decreases your refund, TurboTax is not going to "fix the difference"

[VolvoGirl]

100% Calculation Guarantee, read it yourself

https://ttlc.intuit.com/community/charges-and-fees/help/what-is-the-turbotax-100-accurate-calculatio...

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.