TurboTax does guarantee our calculations to be 100% correct. You should first compare your Notice of Assessment (NOA) with the PDF file of your TurboTax return you filed and see where the discrepancy is. (Your NOA may also tell you if there was a change made to what you filed). Often times there is an extra tax slip out there (T4 from a little job you missed, or T4E for EI benefits you thought were paid in the prior year), and CRA will add this to your tax return when they assess it.
If you do feel that this a calculation error on TurboTax's part, contact TurboTax support team: Contact Us and our guarantee will apply.
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