The turbotax error is that Line10 column (a) of form 2210 is supposed to equal line 27 column (a) of Section AI when using the annualized income installment method. Instead Line 10 column (a) is line 27 column (a) + line 27 column (b).
One would think TurboTax would be highly motivated to fix this error because it can very substantially increase underpayment tax penalties.
