I don't think there is an error in TurboTax.
Sometimes your tax due can increase when your income decreases, as can be the case if you have an earned income credit (EIC). The EIC increases as your income increases up to a point, so if you add expenses on your return and your income decreases as a consequence, it can make your tax due increase.
You can see if you have an earned income credit by seeing if it is listed on line 27(a) of your form 1040.
You can view your form 1040 while working in the online version of TurboTax by following these steps:
While working on your return in the Federal section of TurboTax:
1. Choose the Tax Tools icon on your left menu bar
2. Tools
3. View Tax Summary
4. Choose the Preview my 1040 on your left menu bar
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