If I were to receive communication from the IRS saying that I owe extra taxes, I would compare my 1040 line by line until I can exactly track where I forgot to

If I were to receive communication from the IRS saying that I owe extra taxes, I would compare my 1040 line by line until I can exactly track where I forgot to include the amount the IRS is asking. You are painting it as if they were black box calculations: they are not, especially for "traditional" tax situations it's very feasible to follow the forms (automatically filled by turbotax) and understand where your return differs. That being said, I agree with you that it would be nice if we were directly "billed" by the IRS, but what I wanted to understand from you, and reason of my question, was __exactly__ where TurboTax failed (e.g. exact line of the form or "Oh, I forgot to include a 1099INT via SnapTax that was reported to the IRS"), to get a better perspective of the specific issue. I've been using TurboTax for 10 years with a fairly elaborate tax situation (W2, multiple 1099INT, multiple 1099B, AMT tax for startup equity, AMT credits, backdoor Roth IRA, ...) and I've always been able to get the number right to the point that I never received any further bills from IRS (though I might get audited in the future, obviously).