Is the turbotax software at fault for not catching an error on my 2018 tax return, in which my taxable retirement income was incorrectly reported?

If the box 2a on the 1099R doesn't have an amount and/or the box 2b is checked then the program will calculate the taxable portion based on your data entries in the following interview screens.   Another data input error that happens a lot is if the box 2a is either blank or has words like "unknown" and the user enters a zero by mistake.  

If you download the amendment program then you can review all the data entry screens to see if the mistake was yours.  See this TurboTax FAQ for help with amending:

https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed

TurboTax guarantees the accuracy of the calculations on the tax return.  It cannot and does not guarantee that the tax data entered by the user OR imported to the software is accurate.

You always owe your correct income tax.  At most, the accuracy guarantee would cover any penalties and interest due to a program mistake.  You must file your claim within 30 days of receiving the IRS notice.

See this TurboTax support FAQ for the Accuracy Guarantee - https://ttlc.intuit.com/community/charges-and-fees/help/what-is-the-turbotax-100-accurate-calculatio...