... just to clarify, prior to making the change, I had made it all the way to the end of tax prep and had successfully passed review process and there was no penalty. The penalty did not appear until after I temporarily made the change and then switched back. Seems like a bug/error in Turbotax, but there's no way to get rid of it??? Am i going to have to delete everything and start over to get rid of it?
I would not clear and start over. You either owe a penalty for underpayment of estimated tax, or you don't. TurboTax would not calculate a penalty if you didn't have an underpayment and have tax due. Switching between the Standard Deduction and itemized deductions would not trigger a penalty calculation. I can't explain why you didn't see it before switching between itemized and the standard deduction. The IRS will calculate any underpayment penalty you may have, whether you include it on your tax return or not. If you include one, and don't owe it, the IRS will send you a refund. I would trust the program and go ahead and file.