Most CRA reassessments come from a handful of repeat mistakes — not complex tax planning gone wrong. Here's what trips up Canadian filers most often.
Forgetting a T4, T4A, or T5 slip
The CRA receives copies of every slip issued in your name directly from the payer. If you leave one off your return — a small T5 from a bank account you forgot about, for instance — the mismatch is caught automatically and typically triggers a reassessment months later.
Over-contributing to your RRSP
You get a lifetime cushion of $2,000 in excess contributions before penalties kick in, but beyond that, over-contributions are taxed at 1% per month until withdrawn. Always check your RRSP deduction limit on your latest Notice of Assessment before contributing near the cap.
Claiming home office expenses incorrectly
The temporary flat-rate method used during the pandemic years is no longer available. Employees claiming home office expenses now need a completed Form T2200 from their employer and must use the detailed method, calculating actual costs based on the workspace's share of the home.
Mixing up the filing deadline and the payment deadline
Self-employed individuals get until June 15 to file, but any balance owing is still due April 30 — filing on the later deadline doesn't stop interest from accruing on unpaid tax starting May 1.
Not reporting the sale of your principal residence
Even when the gain is fully exempt, selling your principal residence still needs to be reported on Schedule 3. Skipping this can mean losing the exemption altogether, and it's an easy thing to overlook since no tax is actually owed on the sale itself.
Guessing instead of using Auto-fill my return
If you're filing with NETFILE-certified software and have a CRA My Account set up, Auto-fill my return pulls your slips directly from CRA records — removing most of the risk of a transcription error or a forgotten slip in the first place.
Get an accurate estimate before you file.
Use the Income Tax Calculator →If you already made a mistake
You can correct a filed return using Change my Return in CRA My Account, or by mailing a T1-ADJ form. Wait until you've received your original Notice of Assessment before requesting a change — adjusting too early can cause processing delays.
This covers common mechanical errors, not tax planning strategy. If you've received a CRA reassessment or believe you made a significant error, consult a CPA.