Deadlines, Penalties and Official Sources
Common Canadian tax filing mistakes, what they cost and the official sources to check
The personal tax mistakes that most often lead to penalties, interest or lost benefits in Canada, how to fix each one, and the official CRA and Revenu Québec sources to rely on.
The most expensive personal tax mistakes in Canada are rarely about complex rules. They are filing late with a balance owing (5% penalty plus 1% a month), leaving a slip off the return, over-contributing to a TFSA, RRSP or FHSA (1% a month), ignoring instalment reminders, not filing at all when income is low (which stops the Canada Child Benefit and other benefits) and missing the 90-day objection deadline. Each has a known fix. The official sources to check are the Canada Revenue Agency, Revenu Québec in Quebec, the Department of Finance for new measures and the Justice Laws website for the Income Tax Act.
This hub links to every guide in this section. It is general information, not tax or legal advice.
The mistakes, their cost and the fix
| Mistake | Typical consequence | What to do | Guide |
|---|---|---|---|
| Filing after April 30, 2027 with a balance owing | 5% of the balance + 1% per full month late (up to 12 months), plus interest | File on time even if you cannot pay; arrange a payment plan | Late-filing penalty and interest |
| Not filing because income was low | No CCB, Canada Groceries and Essentials Benefit, GIS or provincial credits | File every year, even with zero income | CCB calculation |
| Missing a T4, T5 or T3 slip | Reassessment with interest; repeated omissions within four years can add a penalty | Check CRA My Account slips before filing; request a change if you already filed | Fixing a return |
| Ignoring instalment reminders | Instalment interest, and a penalty if the interest is large | Pay the reminder amounts or calculate your own | Tax instalments |
| TFSA over-contribution (often after a withdrawal and re-deposit in the same year) | 1% a month on the highest excess | Withdraw the excess immediately; file the RC243 return | Over-contribution penalties |
| RRSP over-contribution above the $2,000 buffer | 1% a month | Withdraw the excess; file T1-OVP | Same guide |
| Missing the RRSP deadline (March 1, 2027) | Deduction moves to the next year | Contribute in the first 60 days next time | Deadlines for the 2026 tax year |
| Self-employed paying on June 15 | Interest from May 1 on the balance | Pay by April 30 even though filing is due June 15 | Same guide |
| Missing the 90-day objection window | Loss of the right to appeal | Object within 90 days of the notice, or ask for an extension within a year | Fixing a return |
| Not electing pension splitting or claiming credits | Overpaying tax | Ask for a change for up to 10 previous years | Pension splitting |
Mistakes in calculations people make by hand
- Using the 2025 rate (14.5%) or the old 15% rate for 2026 credits. For 2026 it is 14%; see federal brackets.
- Valuing a deduction at the average rate instead of the marginal rate; see marginal vs average rate.
- Reporting the full capital gain instead of half, or forgetting reinvested distributions in the cost base; see capital gains.
- Claiming medical expenses on the higher-income spouse, where the 3% threshold is larger.
- Assuming the CRA's Auto-fill includes everything. It does not include receipts and may miss late slips.
Official sources and what each is for
| Source | Use it for |
|---|---|
| Canada Revenue Agency (canada.ca/revenue-agency) | Rates, credits, forms, My Account, payment options, payment arrangements, objections, taxpayer relief |
| Revenu Québec (revenuquebec.ca) | Quebec TP-1, Quebec credits, Quebec instalments and penalties, My Account for individuals |
| Department of Finance Canada | Budgets and proposed measures (which only apply once enacted) |
| Justice Laws Website | The Income Tax Act and regulations, current consolidated text |
| Office of the Taxpayers' Ombudsperson | Complaints about CRA service once you have tried the CRA's own complaint process |
| Tax Court of Canada | Appeals after an objection is decided (or 90 days pass without a decision) |
| Financial Consumer Agency of Canada | Plain-language information on registered accounts and financial products |
| Community Volunteer Income Tax Program (and the Quebec volunteer program) | Free return preparation for people with modest income and simple situations |
The CRA's line-by-line guidance in the "Completing a tax return" pages is the most precise free explanation of each line. Secondary sources such as news articles and firm newsletters are useful for summaries but can carry errors or describe proposals as if they were law.
Scams and unreliable advice
- The CRA does not demand immediate payment by gift cards, prepaid cards or cryptocurrency, threaten arrest by phone, or send refunds by text message link. Verify any contact by logging in to My Account directly or calling the number on canada.ca.
- Be wary of preparers who promise a refund before seeing your slips, base their fee on the size of your refund, or suggest claims you do not have receipts for. You remain responsible for what is filed in your name.
- Programs offering donation receipts worth far more than what you pay, or "tax shelters" promising refunds larger than your investment, have repeatedly been reassessed by the CRA.
When to get professional help
Consider a CPA or a tax lawyer when: you received a reassessment or review letter involving several years; you have unreported foreign income or assets (Form T1135 applies when the total cost of specified foreign property exceeds $100,000); you are dealing with a deceased person's final return; you sold a business, rental or farm property; or you are thinking about a voluntary disclosure. Ask for a written engagement letter with the scope and fee.
Frequently asked questions
What is the single most important habit?
File on time every year, even if you cannot pay and even if you had no income. Most penalties and lost benefits follow from not filing.
How long should I keep records?
Generally six years from the end of the tax year to which they relate, longer if you filed late or have an objection open. Keep receipts even when you file electronically.
Can I trust tax software?
Certified software calculates correctly from what you enter. The common errors are input errors and missing slips, which software cannot detect.
Where do I check Quebec rules?
Only on Revenu Québec's site and forms; CRA pages refer Quebec residents there for provincial matters.
Official sources
- Canada Revenue Agency — rates, forms, My Account and objections
- Revenu Québec — the Quebec return
- Justice Laws Website — the Income Tax Act
- Department of Finance Canada — budgets and proposals
- Office of the Taxpayers' Ombudsperson — service complaints about the CRA
- Tax Court of Canada — appeals
