I Used TurboTax last year. This year TurboTax is adding a penalty for underpayment. The IRS didn't see it that way. You should be responsible for any penalties

If I am incurring a penalty for underpayment due to your error - you should be paying it under the guarantee.

Because the IRS didn't flag it I cannot file a claim for giving me inaccurate results. 

The is wrong.