That's not impossible.
Take a look at your tax calculation. If you have a good deal of preferential rate income, such as qualified dividends or long term capital gains, a 0% rate is possible.
Also, its highly unlikely that Turbo Tax has tax wrong.
You can see your method here (you can also see when you print the worksheets for your return):
- My Account
- Tools
- View Tax Summary
- Preview 1040
- look for Tax Smart Worksheet below line 44. Is it Qualified Dividends and Capital Gains worksheet or something other than tax tables?
You can find a user-friendly rundown of tax rates on many investment sites, including the one below:
http://www.schwab.com/public/schwab/nn/articles/Taxes-Whats-New