Last-Minute 2025 T1 Filing: 10 Common Mistakes to Check Before April 30 2026

With the April 30, 2026 T1 deadline roughly a week away, most last-minute filers are rushing through NETFILE software to get their 2025 return in on time. That rush is where the mistakes happen — some cost you a refund, some trigger a CRA reassessment months later, and a few can quietly cost you hundreds of dollars you never notice.

Here are the 10 errors we see most often in last-week filings, plus what to check before you hit submit.

1. Missing T-Slips Because They Were Issued Late

CRA’s Auto-fill my return service pulls every slip the agency has on file for you, but it only works if the issuer has sent the slip to CRA — and some issuers (small employers, credit unions, small investment advisers) file T4s, T5s, and T3s right up to the last legal deadline.

Check: Log in to CRA My Account → Tax Information Slips. Compare against every employer, bank, and broker you had income from in 2025. If a slip is missing from CRA but you have the paper copy, enter it manually — CRA will still match against issuer data and flag the mismatch if you skip it.

What it costs you: Missing a single T4 will trigger a reassessment with arrears interest running from April 30.

2. Wrong Province of Residence (December 31 Rule)

Your province of residence is whichever province you lived in on December 31, 2025 — not where you earned most of your income. If you moved provinces mid-year, your entire 2025 tax is calculated at the new province’s rates.

Check: If you moved in 2025, confirm your province on the T1 matches your December 31 address. Moving from Ontario to Alberta in November 2025 saves thousands compared to staying in Ontario — but you have to actually claim Alberta on the return.

What it costs you: Getting this wrong in either direction either overpays by thousands (ON vs AB on a $100k income is ~$3,800 difference) or triggers a later CRA reassessment.

3. Forgetting RRSP Contributions Made in the First 60 Days of 2026

RRSP contributions made between January 1 and March 3, 2026 (the 60-day window) can be deducted on either your 2025 or 2026 return. Many filers forget to claim the 2026-first-60-days contributions on their 2025 T1 even though doing so usually gives a bigger refund.

Check: Your RRSP contribution receipts from financial institutions have two sections — “first 60 days” and “rest of year.” The first 60 days of 2026 belong on Schedule 7 of your 2025 T1.

What it costs you: A $5,000 contribution you forget to deduct costs you around $1,500 in refund (assuming a 30% marginal rate).

4. Medical and Dental Receipts Left in a Drawer

The medical expense tax credit covers receipts for any 12-month period ending in 2025 — not just the calendar year. This flexibility is widely underused.

Check: Pull receipts for the lowest-income spouse (the credit is usually claimed by the lower-earning partner because the 3%-of-net-income threshold is lower). Include prescriptions, dental, vision, physio, massage (if prescribed), travel for medical care over 40 km, and health premiums from pay stubs. The Medical Expense Calculator checks whether your total clears the 3% threshold.

What it costs you: The threshold for 2025 is $2,834 or 3% of net income (whichever is lower). A family claiming $4,000 of medical expenses on the lower-earning spouse’s return typically gets back $300–$450.

5. Wrong Marital Status After a Separation

Marital status as of December 31, 2025 determines whether you file as single, married, common-law, separated, divorced, or widowed. CRA’s definition of “separated” requires a 90-day separation — living apart for 60 days in November and December doesn’t qualify.

Check: If you separated in October 2025 or earlier, you were separated on December 31. If you separated after October 2, your status is still married or common-law for 2025. This affects spousal credits, GST/HST credit, and Canada Child Benefit calculations.

What it costs you: Wrong marital status is one of the most common CRA reassessments — and it can retroactively claw back benefit payments.

6. Not Claiming Home Office Expenses (Even If You Qualify)

The detailed method (T2200 form required from employer) is still available for 2025. The temporary flat-rate method ($2/day, max $500) that existed during COVID was removed after 2022 — many filers still think it exists and forget to ask their employer for a T2200.

Check: If you worked from home more than 50% of the time for at least four consecutive weeks in 2025, ask your employer to complete a T2200 (or T2200S for a simplified version). Claim a portion of rent, heat, electricity, internet, and (for commissioned employees) property tax and home insurance based on the square footage of your workspace.

What it costs you: A tenant working from a 200 sq ft home office in a 1,000 sq ft apartment paying $2,000/month rent can deduct roughly $4,800/year — worth about $1,400 in refund at a 30% marginal rate.

7. Duplicating the Basic Personal Amount on Pension Income Splitting

If you split pension income with a spouse using Form T1032, the receiving spouse gets the split amount added to their income and a corresponding deduction for the transferring spouse. Some filers forget to complete both sides or miscalculate the optimal split.

Check: Most tax software will optimize this automatically if you enter both spouses’ returns together. If you’re filing separately using paper or simple software, run both returns twice (with and without splitting) at several different split percentages — the optimum is usually 40-50% transferred to the lower-income spouse.

What it costs you: Failing to split pension income on a $60,000 pension between spouses in different brackets can cost $2,000–$4,000/year in combined tax.

8. Forgetting Foreign Property > $100,000 CAD (T1135)

If you held specified foreign property with a total cost basis over $100,000 CAD at any time in 2025, you must file Form T1135 along with your T1. This includes foreign bank accounts, foreign stocks (even held in a Canadian brokerage), foreign rental property, and cryptocurrency held on non-Canadian exchanges.

Check: Review brokerage statements for non-Canadian-domiciled stocks, any foreign bank accounts, and crypto wallet histories. The threshold is based on cost, not market value, and applies at any time during the year.

What it costs you: The penalty for not filing T1135 is $25 per day, minimum $100, maximum $2,500 per year. Repeated non-compliance can trigger penalties up to $12,000.

9. Missed Charitable Donation Carryforwards

Donations made in the current year plus any from the past 5 years can be claimed on your 2025 return. Many filers claim only the current year and forget sitting-on-the-shelf carryforwards from 2020-2024.

Check: Pull your 2024 Notice of Assessment to see the unused donation carryforward balance. Combine it with 2025 donation receipts. The federal credit jumps from 15% to 33% on donations above $200, so combining 5 years of donations into one claim often maximizes the refund.

What it costs you: $1,000 in forgotten carryforwards costs you about $430 in combined federal + provincial credit (depending on province).

10. Not Reviewing CRA’s Auto-Calculated Installments for 2026

If your balance owing on the 2025 return exceeds $3,000 (federal + provincial) and also did in either 2024 or 2023, CRA will require quarterly instalment payments for 2026 starting March 15, 2026. Many filers ignore the instalment reminder until they get a late-instalment interest charge in early 2027.

Check: If your 2025 T1 shows a balance owing of more than $3,000, expect a CRA instalment reminder for 2026. You can either pay the CRA-calculated amount, the prior-year amount, or make realistic current-year estimates (CRA doesn’t charge interest as long as you meet one of these).

What it costs you: Late instalment interest is currently around 8% annualized, compounded daily.

Final Pre-Submit Checklist

Before you click “submit” on NETFILE or hand the paper return to the post office:

If you find a mistake after filing, you do not need to refile — use ReFILE (for the 3 most recent years) or T1-ADJ (for up to 10 years back) via CRA My Account. A wrong entry discovered early costs nothing beyond the few minutes to correct it; a wrong entry caught by CRA two years later costs interest and potentially penalties.

The goal for the next week is simple: file on time, and file correctly. Everything else can be adjusted later.