Capital Gains: Risks, Failure Modes and Common Mistakes

Mistake 1: The Tax Tail Wagging the Investment Dog

The most common and financially damaging capital gains mistake is holding a deteriorating investment position because selling would trigger a tax bill. The logic feels sound in the abstract: paying taxes reduces wealth, so deferring taxes preserves wealth. In practice, this reasoning frequently causes investors to hold positions far past the point where the investment case justified ownership.

The fundamental error is treating the tax as equivalent in magnitude to the position's potential future loss. The tax on a capital gain is a fraction of the gain itself. A long-term capital gains rate of 20% on a $100,000 gain costs $20,000 in tax. If holding the position allows it to decline by $30,000, the investor has "saved" $6,000 in taxes ($30,000 times 20% = $6,000 less tax on the reduced gain) while losing $30,000 in actual value. The net outcome is $24,000 worse than selling at the original gain and paying the tax.

This mistake compounds in specific scenarios. An investor holding a single stock (an executive at their employer company, or an early investor in a successful startup) may watch a position that represents a large fraction of their net worth decline substantially because the embedded gain feels too costly to recognize. The concentrated position carries idiosyncratic risk that diversification would reduce. The tax cost of diversifying is known and fixed; the cost of a catastrophic decline in a concentrated position is unknown and potentially much larger.

The correct framing is: if taxes did not exist, what would I do with this position? If the honest answer is "sell it," then paying the tax and selling is almost certainly the right decision. The tax should inform the timing of the decision at the margin (waiting a few more weeks to reach long-term status, or selling in a lower-income year), but it should not override a clear investment judgment.

Mistake 2: Violating the Wash-Sale Rule

IRC Section 1091 disallows a capital loss deduction when the investor purchases a "substantially identical" security within 30 days before or after the sale that generated the loss. The rule applies to a 61-day window: 30 days before the sale, the day of the sale, and 30 days after. Buying back within this window triggers the rule regardless of which end of the window the purchase falls on.

The disallowed loss is not permanently eliminated. It is added to the cost basis of the repurchased security, deferring the tax benefit until that security is later sold in a non-wash-sale transaction. In this sense, the wash-sale rule is a timing rule, not a permanent disallowance, except when the year ends while you still hold the repurchased security: in that case, the loss benefit is pushed into the next year when the replacement position is eventually sold.

Common violations:

• Selling an S&P 500 index fund at a loss and immediately buying another S&P 500 index fund tracking the same index from a different provider. The IRS has not issued definitive guidance on ETF-to-ETF exchanges within the same index, but many practitioners treat them as substantially identical.
• Reinvesting a dividend or a distribution in the same fund within 30 days of a loss sale of that fund. Automatic dividend reinvestment programs (DRIPs) can trigger this inadvertently.
• Selling a position at a loss in a taxable account while holding or buying the same security in an IRA or 401(k) during the 61-day window. The wash-sale rule applies across all accounts owned by the same taxpayer, including retirement accounts, though the disallowed loss in a wash-sale involving a retirement account is permanently lost (not added to the retirement account's basis).

Working around the rule: The standard approach is to buy a related but not substantially identical position during the 30-day window to maintain market exposure. Selling an S&P 500 ETF and buying a total U.S. stock market ETF (which includes small and mid-cap stocks not in the S&P 500) is generally treated as a permissible substitute. After 31 days, you can switch back to the original position if preferred.

Mistake 3: Ignoring State Capital Gains Taxes

State income tax is a large and frequently underweighted component of after-tax capital gains calculations. For investors in high-tax states, the state component can be nearly as large as the federal component on short-term gains, and it applies equally to long-term gains in states that do not differentiate by holding period.

High-tax state examples:

• California: Taxes all capital gains as ordinary income, with a top rate of 13.3% (12.3% plus the 1% Mental Health Services Tax on income above $1,000,000). For a California investor with a short-term gain in the 37% federal bracket, the combined marginal rate is 37% + 3.8% NIIT + 13.3% state = 54.1%. Even for long-term gains, the combined rate is 20% + 3.8% NIIT + 13.3% state = 37.1%.
• New York State plus New York City: State rate up to 10.9%; NYC adds up to 3.876%. Combined state and local top rate of approximately 14.8% for NYC residents, on top of federal rates.
• Oregon: Top rate of 9.9% on all capital gains, no preferential rate for long-term gains.

Zero-tax states: Texas, Florida, Nevada, Washington, Wyoming, South Dakota, and Alaska impose no personal income tax. A Texas investor selling a long-term position in the 20% federal bracket pays only 20% + 3.8% NIIT = 23.8% on the gain. A California investor with the same income profile pays 20% + 3.8% + 13.3% = approximately 37.1%. That 13.3 percentage point difference on a $500,000 gain is $66,500.

State residency at the time of the sale determines which state's tax applies. Investors who are planning a move between states should consider the timing of large capital gain realizations relative to their change of domicile. Establishing legal residency in a no-tax state before realizing a large gain is a legitimate tax planning strategy, but it requires genuine change of domicile, not a nominal address change. States like California aggressively audit residents who claim they have moved to avoid taxation while maintaining ties to California.

Mistake 4: Miscounting the Holding Period

The holding period for capital gains purposes begins the day after the trade date of purchase and ends on the day of sale. The day of purchase is day zero; the first day of the holding period is day one. To qualify for long-term capital gains treatment, the sale must occur on day 366 or later (or day 367 in a leap year if the purchase was in the year before the leap year).

An investor who purchases on January 2, 2025 and sells on January 2, 2026 has held for exactly 365 days. Short-term treatment applies. Selling on January 3, 2026 (day 366) qualifies for long-term rates. Missing this by one day on a large position is a costly error that brokerages will not catch for you; they report the holding period but do not prevent the sale.

Special holding period rules that many investors get wrong:

• Gifted securities: The recipient of a gift (where the gift is not subject to gift tax paid by the donor) takes over the donor's original holding period. If the donor bought the stock 2 years ago and gifts it today, the recipient's holding period begins at the donor's original purchase date. The recipient immediately qualifies for long-term rates if they sell.
• Inherited securities: Regardless of how long the heir actually holds the inherited security, it automatically qualifies for long-term capital gains rates when sold. There is no minimum holding period requirement for inherited property. The heir receives a stepped-up basis and long-term status simultaneously at the time of inheritance.
• Employee stock options (incentive stock options and non-qualified stock options): The holding period for shares received from exercising options begins on the exercise date, not on the date the options were granted. An investor who was granted options 5 years ago and exercised them yesterday has a zero-day holding period on the shares, not a 5-year period. For ISO shares, there are additional rules requiring a hold of at least one year after exercise and two years after grant to receive long-term treatment and avoid ordinary income recognition at exercise.
• Short sales: A short sale is considered to close (and the holding period to begin) on the date the borrowed shares are returned, not the date of the original short sale. Special constructive sale rules also apply for certain hedging arrangements.

Frequently Asked Questions

References

• IRS: Publication 550, Investment Income and Expenses (wash-sale rules at Section 1091)
• IRS: Topic 409, Capital Gains and Losses