Top 10 Common Tax Filing Mistakes to Avoid in 2026

Filing taxes is complicated, and even simple errors can lead to processing delays or unwanted attention from the CRA. Here are the most common pitfalls to watch out for, based on CRA audit data and tax professional insights.

The Top 10 Tax Filing Mistakes

Filing Too Early (Missing Slips): This is the #1 error. If you file in February but a T3 or T5 slip arrives in March, you have to file an amendment (T1-ADJ). It creates a mess. Wait until mid-March to ensure you have ALL your slips, including those from investments which often arrive late.
Prevention Tip: Log into CRA My Account and use the "Auto-fill my return" feature, which shows all T-slips the CRA has received for you. If you see slips there that you haven't received by mail, download them directly.
Guessing Amounts: The CRA receives copies of all your T-slips. If you estimate your income and it's off by even a few dollars, their system will flag it for manual review. Always use the exact numbers from your T3, T4, and T5 slips.
Real Example: Sarah estimated her T4 income as $50,000 when it was actually $50,127.43. Her return was delayed by 8 weeks for manual verification, delaying her refund.
Forgetting Carry-Forwards: Check your Notice of Assessment (NOA) from last year. You may have unused tuition credits, capital losses, or charitable donations waiting to be used. These "banked" credits can save you thousands.
Common Scenario: Students who graduated 3 years ago often have $20,000+ in unused tuition credits that they forget to claim once they start earning a good salary.
Not Reporting Foreign Income: Many Canadians forget that US dividends (like from Apple or Microsoft stock) in a non-registered account must be reported in Canadian dollars. The CRA receives data from foreign tax treaties, so they know if you missed it.
Important: You must also file Form T1135 if your foreign property cost more than $100,000 CAD at any time during the year. Failure to file can result in penalties of $2,500 or more.
Missing Medical Expenses: People often forget they can claim travel insurance, premiums deducted from paycheques, and gluten-free food (for Celiac disease with a doctor's note). You can substantially lower your tax bill by grouping these expenses on the return of the spouse with the lower net income.
Often Overlooked: Prescription sunglasses, laser eye surgery, fertility treatments, and private health insurance premiums are all eligible.
Over-Contributing to RRSPs: Check your specific contribution room on your NOA. The penalty for over-contribution is 1% per month on the excess amount over $2,000. This penalty adds up fast and is not tax-deductible.
Warning: If you have a company pension plan, your "Pension Adjustment" (PA) reduces your RRSP room significantly. Always check before contributing.
Improperly Claiming Moving Expenses: Ensure you moved at least 40km closer to work or school. Also, remember you can only deduct moving expenses against income earned at the NEW location.
Key Rule: If you moved for school, you can only claim expenses against scholarships, grants, or employment income earned while at school.
Confusing the Canada Caregiver Credit: Ensure your dependent actually qualifies (infirmity requirement). Providing basic financial support to an elderly parent isn't enough; they must be dependent on you due to a physical or mental impairment.
Documentation Required: You may need a signed statement from a medical practitioner confirming the impairment.
Not Keeping Receipts: If you claim it, you must be able to prove it. The CRA can ask for receipts up to 6 years later. Digital copies are acceptable, so scan everything.
Best Practice: Use a cloud storage service (Google Drive, Dropbox) to store scanned receipts organized by tax year. This makes retrieval easy during an audit.
Ignoring CRA Mail: If the CRA asks for more info, respond immediately. Ignoring them often leads to them simply denying the claim and reassessing you with interest and penalties.
Time Limit: The CRA typically gives you 30 days to respond. If you need more time, call them immediately to request an extension.

Bonus Mistake: Marital Status Changes

Failing to update your marital status to "Separated" or "Divorced" is a common error. Updating this can often increase your benefits (GST/HST Credit, Canada Child Benefit), so do it as soon as the 90-day separation period is over.

How to Update: Log into CRA My Account and select "Change my marital status" under the "Personal info" section. You must update this within one month of the change to avoid potential penalties.

How to Fix Mistakes After Filing

If you discover an error after filing, don't panic. You have several options:

Online: Use "Change my return" in CRA My Account (available 2-3 weeks after filing)
By Mail: Complete Form T1-ADJ (T1 Adjustment Request)
By Phone: Call 1-800-959-8281 for simple changes
ReFILE: If you used NETFILE originally, you can use ReFILE to submit adjustments electronically

You have up to 10 years from the end of the tax year to request most adjustments, but it's best to fix errors as soon as you discover them.

Key Takeaways

Wait until mid-March to file to ensure you have all tax slips
Use exact numbers from your T-slips—never estimate
Check your Notice of Assessment for carry-forward amounts
Report all foreign income and file T1135 if required
Keep all receipts for 6 years, preferably as digital copies
Respond to CRA requests within 30 days
Update marital status changes within one month

Frequently Asked Questions

Q: What happens if I make a mistake on my return?
A: If the CRA catches it first, they'll send you a Notice of Reassessment showing the changes. If you catch it first, you can file an adjustment request to correct it. Minor errors usually don't result in penalties if corrected promptly.

Q: Can I be audited for making honest mistakes?
A: Honest mistakes rarely trigger audits. The CRA distinguishes between errors and intentional tax evasion. However, repeated errors or patterns of claiming unusual deductions may increase audit risk.

Q: How long does it take to process an adjustment?
A: Online adjustments through CRA My Account typically process within 2 weeks. Paper adjustments (Form T1-ADJ) can take 8-12 weeks.