If you filed an extension for your 2025 return, this guide explains the most common capital gains mistakes U.S. taxpayers make when they sell stocks, ETFs, mutual funds, and similar investments. It focuses on the federal rules for individual filers and shows where the IRS expects you to correct basis, wash sales, and holding-period errors on Form 8949 and Schedule D (Form 1040).
Quick Takeaways
- For a 2025 return filed in 2026, most individual filers who requested an extension had until October 15, 2026 to file, but the tax payment deadline was still April 15, 2026.
- Your cost basis is generally what you paid for the stock or fund plus purchase costs like commissions and transfer fees, so a broker’s number may not be the final number.
- A wash sale can disallow a loss if you buy substantially identical stock or securities within 30 days before or after the sale. The IRS says you cannot deduct that loss even if your broker does not report it.
- If you need to adjust broker-reported proceeds, basis, or wash-sale amounts, the IRS expects you to use Form 8949 and then carry the totals to Schedule D.
- For most stock sales, a gain or loss is short-term if you held the shares 1 year or less and long-term if you held them more than 1 year.
Who This Applies To
This article is mainly for individual taxpayers who report investment sales on Form 1040. State filing rules can differ, so check your state return separately before copying federal numbers. The IRS also notes that corporations and partnerships use Form 8949 for certain capital-asset transactions, so business returns and dealer activity can follow a different workflow.
Introduction
Capital gains reporting looks simple until the details show up: the wrong basis, an accidental wash sale, the wrong holding period, or a broker statement that does not match your own records. For a 2025 return filed in 2026, those mistakes matter because the extension only gives you more time to file, not more time to pay.
This guide focuses on common filing mistakes for stock and fund sales. It does not try to cover every capital-asset rule, and it does not replace personalized tax advice. If your return has lots of trades, multiple brokerage accounts, spouse-level transactions, or corrected statements, the facts matter.
What the Terms Mean
Cost basis
Your cost basis is generally your purchase price plus purchase costs such as commissions and transfer fees. That number is the starting point for figuring gain or loss, but it can change over time if there are adjustments, such as certain corporate actions or other basis rules.
Wash sale
A wash sale happens when you sell stock or securities at a loss and, within 30 days before or after the sale, you buy substantially identical stock or securities, acquire them in a fully taxable trade, or buy a contract or option to buy them. The IRS also says a loss can be disallowed if your spouse or a corporation you control buys the same or substantially identical position. If the wash sale rule applies, the loss is generally added to the basis of the replacement stock. Whether two securities are “substantially identical” depends on the facts and circumstances.
Short-term vs. long-term
The holding period matters. For most stock sales, the IRS treats the gain or loss as short-term if you held the shares for 1 year or less and long-term if you held them for more than 1 year. That determines how you report the sale and can affect the tax result.
What Matters for 2025
For a 2025 return, the practical issue is accuracy, not a new set of special rules. Use the 2025 versions of Form 8949, Schedule D, and Publication 550, and reconcile the broker’s reporting with your actual records before you file. The IRS instructions still expect you to fix basis errors, wash-sale adjustments, and short-term versus long-term classification on the return.
The Most Common Filing Mistakes
1) Copying the broker’s basis without checking it
This is the most common error. Your broker’s Form 1099-B basis is a starting point, not a guarantee. The IRS says stock basis is generally purchase price plus purchase costs, and the 2025 Form 8949 instructions say that if the basis reported to the IRS is wrong, you still report the transaction on Form 8949 and make the correction there.
2) Missing a wash sale because it happened in another account
Wash sales are easy to miss when you trade the same security in more than one account or when a spouse buys the same name. The IRS says you cannot deduct a wash-sale loss even if it is not reported on Form 1099-B. On Form 8949, you enter code W in column (f) and the disallowed loss as a positive adjustment in column (g).
3) Putting a trade in the wrong holding-period bucket
A sale that is short-term is not long-term, even if the difference is just one day. For most stock sales, the IRS says more than 1 year is long-term and 1 year or less is short-term. Put the sale in the right part of Form 8949, then carry the totals to Schedule D.
4) Using the wrong lot method for shares bought at different times
If you bought the same security in several lots, the tax result depends on which shares you sold. The IRS says you can use specific share identification if you adequately identify the shares sold; otherwise, FIFO applies. For mutual funds and certain dividend reinvestment plans, average basis may be allowed. The key is to give the broker instructions at the time of sale and keep written confirmation.
5) Skipping Form 8949 when you need it — or using it when you do not
Form 8949 is the IRS reconciliation form for broker-reported sales. If basis was reported to the IRS and you do not need any adjustments, the IRS says you may be able to summarize those transactions directly on Schedule D. But if basis, proceeds, or wash-sale amounts need correction, Form 8949 is the right place to make the adjustment.
6) Forgetting that state returns can differ
Your federal answer does not automatically control your state return. Some states follow the federal treatment closely; others do not. Always check the state instructions before assuming the federal basis or gain amount is the one you should use on the state return.
How to Report the Return Correctly
A clean filing process usually looks like this:
- Start with every Form 1099-B and compare it to your purchase confirmations and trade history.
- Separate short-term from long-term sales before you start the return.
- Fix basis errors where the broker’s number does not match your records.
- Mark wash sales with code W and enter the disallowed amount in column (g).
- Carry totals to Schedule D only after Form 8949 is complete.
If you use specific identification, keep the broker’s written confirmation. If you use FIFO, keep records showing why that method applies. For mutual funds, keep the election and basis records together with the return file.
How This Differs for Individuals vs. Businesses
This article is written for individual investors filing Form 1040. Business returns can be different. The IRS instructions note that corporations and partnerships use Form 8949 for certain capital-asset reporting, and Pub. 550 says the wash sale rules do not apply to a dealer in stock or securities when the loss is from a transaction made in the ordinary course of business. If you file through a partnership, corporation, or dealer activity, use the return-specific instructions rather than assuming the individual workflow applies one-for-one.
Myth vs. Fact
- Myth: If my broker does not show a wash sale, I can claim the loss. Fact: The IRS says a wash-sale loss is not deductible even if it is not reported on Form 1099-B.
- Myth: The broker’s basis is always the final tax basis. Fact: If the basis is wrong or incomplete, you are responsible for correcting it on Form 8949.
Practical Examples
Simplified illustration 1: Wrong basis because commission was left out. You buy 100 shares for $50 each, so the stock costs $5,000. You also pay a $40 commission. Your real basis is $5,040. If you sell the shares for $6,100 and copy a broker basis of $5,000, you overstate your gain by $40. The IRS says purchase costs belong in basis, and Form 8949 is where you fix the mismatch.
Simplified illustration 2: Wash sale after tax-loss harvesting. You buy 200 shares for $30 each, or $6,000 total. Later you sell them for $26 each, or $5,200 total, creating an $800 loss. Twelve days later, you buy the same stock back for $25 per share, or $5,000 total. The $800 loss is disallowed as a wash sale, and your replacement basis becomes $5,800. On Form 8949, code W and a positive $800 adjustment show what happened.
Simplified illustration 3: One day changes short-term to long-term. You buy shares on July 10, 2024. If you sell on July 9, 2025, the sale is generally short-term. If you sell on July 11, 2025, the sale is generally long-term. That one-day difference can change where the sale goes on Form 8949 and how it affects your return.
Simplified illustration 4: Multiple lots and the wrong default method. You buy the same mutual fund in several lots at different prices. If you do not adequately identify which shares you sold, the IRS says FIFO applies. For mutual funds and certain dividend reinvestment plans, average basis may be allowed. That is why lot-level records matter when you sell part of a position.
Checklist Before You File
| Mistake | Why it matters | What to do instead |
|---|---|---|
| Wrong basis on the broker statement | Gains and losses are misreported when commissions or other basis adjustments are missed. | Rebuild basis from trade confirmations and correct it on Form 8949. |
| Missed wash sale | A loss can become nondeductible if you bought substantially identical stock within 30 days before or after the sale. | Use code W on Form 8949 and add the disallowed loss to the replacement basis where applicable. |
| Wrong holding period | Short-term and long-term gains go in different places. | Use the more-than-1-year rule before you file. |
| Wrong share-lot method | FIFO can override the lot you thought you sold. | Use specific ID only with broker instruction and written confirmation; mutual funds may allow average basis. |
| Skipping or overusing Form 8949 | You may omit needed adjustments or create unnecessary line-item reporting. | Use Form 8949 when basis or wash-sale adjustments are needed; summarize on Schedule D only when the instructions allow it. |
Before you hit file
- Pull every Form 1099-B and match it to your own trade history.
- Check every loss sale for a repurchase within 30 days before or after the sale.
- Confirm whether your sale is short-term or long-term.
- Keep lot-by-lot records, especially if you used specific identification or mutual fund average basis.
- Review the state return separately. Federal and state treatment may not match.
FAQ
What is cost basis in plain English?
It is usually what you paid for the stock or fund, plus purchase costs such as commissions and transfer fees. That number is the starting point for figuring gain or loss.
What exactly is a wash sale?
A wash sale generally happens when you sell stock or securities at a loss and buy substantially identical stock or securities within 30 days before or after the sale. The IRS says the loss is not deductible in that situation.
Do I always need Form 8949?
No. If the broker reported basis to the IRS and you do not need any adjustments, the IRS says you may be able to summarize those transactions directly on Schedule D. If basis or wash-sale amounts need correction, use Form 8949.
Can a wash sale happen if my spouse buys the same stock?
Yes. The IRS says a wash sale can also occur if your spouse or a corporation you control buys substantially identical stock or securities.
Does FIFO always apply?
No. If you adequately identify the shares you sold, you can use those shares’ adjusted basis. If you do not identify them properly, FIFO applies. Certain mutual fund and DRIP shares may allow average basis.
Does this article cover state returns too?
Only briefly. State rules can differ, so check your state’s instructions before you copy federal gain, loss, or basis numbers onto the state return.
Bottom Line
The biggest capital gains mistakes are usually simple ones: wrong basis, missed wash sales, and the wrong holding period. For a 2025 return filed in 2026, the safest approach is to match every sale to your records, correct the numbers on Form 8949 when needed, and carry the totals to Schedule D only after the forms are right. If your trades span multiple accounts, spouses, or complex lot histories, a CPA, EA, or tax attorney can help you clean up the return before you file.
What to do next
- Reconcile every 1099-B against your own purchase and sale records.
- Check for wash sales across all accounts and spouse-level trades.
- Verify whether each sale is short-term or long-term.
- Use Form 8949 to fix basis or wash-sale errors before you file.
- Review the state return separately. State treatment may differ.
Source note: Sources consulted: IRS forms, instructions, publications, official updates, and related guidance.
