there are two types of errors that can occur, both result in incorrect tax returns, but the root cause of the incorrect tax return is very, very different
1) the software may be 'buggy'. if the software doesn't correctly reflect the IRS rules, the tax return can be incorrect and could result in penalties and interest for underpaying taxes. The Turbo Tax guarentee covers this situation (but the user is still responsible for the tax underpayment, but TT will cover penalties and interest).
2) the taxpayer enters the information incorrectly. Entering the information incorrectly results in the tax return being incorrect and could result in penalties and interest for underpaying taxes (let alone the underpayment itself). The Turbo Tax guarentee doesn't cover 'user error'.
