What is the most common tax mistake in Canada?

The most common mistake is forgetting to claim eligible deductions and credits, especially: RRSP contributions, medical expenses, charitable donations, tuition credits, moving expenses, and home office deductions. The CRA estimates billions in unclaimed credits each year. Always review the full list of deductions before filing.

What happens if you file your taxes late in Canada?

If you owe taxes and file late, the CRA charges a late-filing penalty of 5% of the balance owing plus 1% per month (up to 12 months) for a maximum of 17%. Repeat late filers face doubled penalties (10% + 2%/month). If you are owed a refund, there is no penalty for filing late, but you should still file within 3 years to claim it.

Can the CRA audit you for mistakes on your tax return?

Yes. The CRA can review and reassess any tax return within 3 years of the original Notice of Assessment (or 6 years in cases of suspected negligence). Common audit triggers include: large charitable donations relative to income, significant changes in income year-over-year, home office claims, and missing T-slips.

Common Tax Mistakes Canadians Make in 2026

CRA estimates that Canadians leave billions in unclaimed credits on the table every year — and the most common mistakes are also the easiest to avoid. Missing medical expenses, forgetting RRSP deductions, skipping the tuition tax credit, and not filing as a couple for optimized credits can cost you hundreds to thousands of dollars per return. On the penalty side, filing late when you owe triggers an automatic 5% charge plus 1% per month, turning a $5,000 tax bill into $5,850 within a year. Use CRA auto-fill through your tax software and review the full list below before you hit submit.

To avoid common filing errors, first lock in dates and inputs with tax deadline Canada and how to read your T slips. Verify outcomes in how to know if your tax return was accepted, then review your assessment with how to read your Notice of Assessment.

Top Tax Mistakes and How to Avoid Them

Filing Mistakes

Mistake	Impact	How to Avoid
Filing late (when you owe)	5% penalty + 1%/month (up to 17%)	Set a reminder for April 30 deadline
Forgetting to file (no balance owing)	Lose refund; interrupts benefit payments (CCB, GST/HST credit)	File even if you owe nothing
Wrong SIN or personal information	Delays processing; may not match CRA records	Double-check before submitting
Missing T-slips	CRA will reassess and may add penalties	Check My Account for all slips by end of March
Not reporting all income sources	CRA matches T-slips; penalties for omission	Report everything, including gig and side income
Using the wrong tax year’s software	Applies incorrect rates and credits	Always use current-year certified software

Missed Deductions and Credits

Commonly Missed Credit/Deduction	Who Qualifies	Typical Value
RRSP contributions	Anyone with contribution room	20–53% refund on contribution
Medical expenses	Over-the-counter and prescribed costs above 3% of income or $2,759	$200–$2,000+
Charitable donations	Anyone donating to registered charities	15–33% credit
Tuition credit (federal)	Post-secondary students	15% of tuition paid
Moving expenses	Moved 40+ km closer to work or school	Cost of move (up to income earned at new location)
Home office deduction	Employees working from home or self-employed	$2/day (simplified) or actual expenses
Child care expenses	Parents with children under 16	Up to $8,000/child under 7
Disability tax credit	Individuals with qualifying impairment	$9,428 (2025) non-refundable credit
Canada Workers Benefit (CWB)	Low-income workers	Up to $1,518 (single)
Student loan interest	Anyone repaying government student loans	15% credit on interest paid
Union/professional dues	Employees who pay union or professional fees	Full deduction
Northern residents deduction	Living in a prescribed northern zone	$11/day (intermediate) to $22/day (northern)

RRSP and TFSA Mistakes

Mistake	Impact	How to Avoid
Over-contributing to RRSP	1%/month penalty on excess over $2,000	Check your limit on My Account before contributing
Not claiming RRSP deduction for past contributions	Paying more tax than necessary	Carry forward and claim in a higher-income year
Contributing to RRSP after age 71	Not allowed (must convert to RRIF by Dec 31 of year you turn 71)	Contribute to spousal RRSP if spouse is under 71
Over-contributing to TFSA	1%/month penalty on excess	Check room on My Account; avoid re-contributing in the same year as a withdrawal
Withdrawing from RRSP to fund short-term needs	Income tax + lost contribution room	Use TFSA or emergency savings instead
Not naming a beneficiary	RRSP/TFSA goes through estate (probate + delays)	Name a beneficiary on the account

Self-Employment and Side Income Mistakes

Mistake	Impact	How to Avoid
Not reporting side income	CRA audits; penalties + interest	Report all income over $500 on T2125
Not collecting GST/HST when required	Penalties; owe back GST/HST	Register once revenue exceeds $30,000 in 4 rolling quarters
Not saving receipts for business expenses	Can’t claim deductions; lost savings	Keep receipts digitally (photos/scans) for 6 years
Not making quarterly instalment payments	Interest charges	Pay quarterly if net tax owing exceeds $3,000
Claiming personal expenses as business	Audit risk; penalties	Only claim expenses with a clear business purpose

Late-Filing and Late-Payment Penalties

Situation	Penalty
Filed late, owe taxes (first offence)	5% of balance + 1%/month (max 12 months)
Filed late, owe taxes (repeat offence)	10% of balance + 2%/month (max 20 months)
Filed late, owe nothing or owed refund	No financial penalty, but delays refund and benefits
Paid late (filed on time)	0% penalty, but compound daily interest on amount owing
Interest rate on unpaid balance	CRA prescribed rate + 4% (currently ~9–10%)
False statement or omission	50% of understated tax or overstated credit (gross negligence penalty)

The penalties for honest mistakes are significant but manageable — the penalties for fraud or gross negligence are career-altering. CRA’s gross negligence penalty is 50% of the tax you tried to avoid, on top of the tax itself and interest. If you realize you made an error on a past return, file an adjustment voluntarily through CRA My Account or Form T1-ADJ. CRA treats voluntary corrections far more leniently than corrections they discover during a review or audit.

The Bottom Line

Use CRA auto-fill to import all your tax slips, file as a couple to optimize credits, and review the commonly missed deductions list above before submitting. The five minutes you spend checking for medical expenses, charitable donations, moving costs, and RRSP carryforwards can save you hundreds of dollars. File on time even if you can’t pay — the late-filing penalty is separate from and in addition to interest on amounts owing. And if you have side income, register for GST/HST before you hit $30,000 in revenue, not after.