Tax-Loss Harvesting Mistakes to Avoid

By Tom, CPA · Updated October 27, 2026

Part of: Tax Planning

TL;DR: The wash sale rule spans all your accounts, including IRAs and your spouse's. Turn off dividend reinvestment, pick a different-index replacement, mind short vs long-term character, and fix errors on Form 8949.

Educational only, not tax, legal, or investment advice. Examples are illustrations, not predictions. Rules and limits change; verify current figures and talk to a qualified professional about your situation.

Tax-loss harvesting is one of the few free lunches in investing: sell a loser, book the loss, buy something similar, and let the IRS share your pain. But the lunch comes with a fine-print menu, and the wash sale rule is the waiter who quietly takes your plate away. The tax code is 4 million words long; you need about 400 of them to avoid the mistakes that turn a harvested loss into a disallowed one.

This checklist covers the common traps: wash sales across accounts, dividend reinvestment, short-term vs long-term mismatches, state issues, and how to fix mistakes. For the strategy itself, start with our tax-loss harvesting guide. Rules are federal, as of Oct 2026.

CPA Warning Label: An IRA purchase can permanently destroy a harvested loss. Pause automatic buys of the same fund everywhere for 31 days.

Mistake 1: wash sales across accounts

A wash sale happens when you sell a security at a loss and, within 30 days before or after the sale, you (or your spouse) buy a "substantially identical" security. The 61-day window includes the sale date. The loss is disallowed for now and added to the basis of the replacement shares.

The trap is that the rule applies across all your accounts, not just the one where you sold:

• Selling an S&P 500 fund at a loss in your brokerage account while your 401(k) buys the same fund with each paycheck.
• Your spouse buying the same stock in their account.
• Buying the replacement in an IRA or Roth IRA. Per IRS Revenue Ruling 2008-5, a wash sale triggered by an IRA purchase disallows the loss and does not add it to the IRA basis, so the loss is effectively gone forever.

Brokers only track wash sales within the same account for the same security. Cross-account wash sales are your job to catch.

Checklist: pause automatic purchases of the same fund in other accounts for 31 days, check your spouse's accounts, and never buy the replacement in an IRA.

Mistake 2: dividend reinvestment traps

Automatic dividend reinvestment buys new shares, which counts as a purchase. If a fund pays a dividend within 30 days of your loss sale and reinvests, a small portion of your loss becomes a wash sale. Usually the damage is limited to the number of shares bought, but it complicates records.

Checklist: turn off dividend reinvestment on the position you plan to harvest, ideally across all accounts holding the same fund, for at least 31 days before and after the sale. Check ex-dividend dates, especially in December when many funds distribute.

Mistake 3: 'substantially identical' guesses

The IRS has not issued a bright-line definition. Selling one company's stock and buying the same company's stock is clearly identical. Selling one S&P 500 index fund and buying another S&P 500 index fund from a different provider is a gray area that many investors accept but that has not been definitively settled. Switching to a fund tracking a different index (for example, total market instead of S&P 500) is a commonly used, more conservative approach.

Checklist: choose a replacement tracking a different index, or wait 31 days, if you want to minimize risk.

Mistake 4: short-term vs long-term mismatches

Capital losses first offset gains of the same character: short-term losses against short-term gains, long-term against long-term. Then net losses of one type offset net gains of the other. Up to $3,000 of net capital loss ($1,500 if married filing separately) can offset ordinary income each year, with the rest carried forward indefinitely. See our capital loss carryforward guide.

Common errors:

• Harvesting a loss in a year when it offsets low-taxed long-term gains, when it could have offset short-term gains or ordinary income taxed at higher rates.
• Harvesting losses in the 0% long-term capital gains bracket, where gains would have been tax-free anyway. You lower your basis for nothing; tax-gain harvesting might fit better.
• Forgetting the basis reset. The replacement has a lower basis, so you have deferred tax, not eliminated it, unless you hold until death or donate the shares.

Mistake 5: state issues

Most states follow federal capital loss rules, but not all treat capital losses or carryforwards the same way. A few states have no income tax, so harvesting saves only federal tax. If you plan to move, a loss realized while living in a high-tax state can be worth more than the same loss realized after moving to a no-tax state. Check your state revenue department's rules for capital loss limits and carryforwards.

Mistake 6: forgetting the rental and business side

Investment capital losses are not the same as rental losses. Passive rental losses follow separate rules, including the $25,000 passive loss allowance, and do not offset capital gains in the same way. Casualty losses on rentals have their own treatment; see rental property casualty loss. Don't assume one kind of loss can absorb another.

Fixes if you already made the mistake

• Taxable-account wash sale: the loss is not gone; it is added to the replacement's basis and the holding period carries over. Sell the replacement later, outside any new wash window, to realize it.
• IRA wash sale: the loss is permanently disallowed. Learn the lesson and move on.
• Broker reporting: Form 1099-B reports wash sales the broker identified. You must adjust for cross-account wash sales yourself on Form 8949.
• Missed carryforward: if you forgot to carry a capital loss forward on a prior return, you may be able to file an amended return; see our 1040-X guide.

Illustration

On November 10, Lin sells a total market index fund in a brokerage account for a $12,000 loss. On November 20, Lin's 401(k) buys $500 of the same fund via payroll. Those shares likely trigger a partial wash sale: the loss on a matching number of shares is disallowed. The IRS has not ruled specifically on 401(k) purchases, but the logic of Rev. Rul. 2008-5 for IRAs is widely applied, which would mean the disallowed slice never comes back as usable basis. Lin should have paused the 401(k) allocation to that fund or picked a different fund in the taxable account. Lin's remaining loss still offsets gains, and up to $3,000 can offset ordinary income. Illustrative only.