How Small Financial Choices Can Lead to Huge Tax Bills

The IRS assessed about $4.8 billion in estimated-tax penalties on more than 15 million individual returns in fiscal year 2024—almost triple what it collected two years earlier. Many of those bills came from underpaying taxes from selling a winning stock, withdrawing retirement funds, or simply failing to talk to a tax professional before making a move.​

“The No. 1 mistake I see with clients is underestimating how many financial decisions have an impact on their taxes,” Brenton D. Harrison, founder of New Money, New Problems, told Investopedia. “I’ve seen clients stuck with six-figure tax bills that could have been avoided if they’d simply consulted with their CPA [certified public accountant] or financial advisor before making a decision.”​

Here are five mistakes that catch people off guard, and how to avoid them.

1. Forgetting About the Tax Implications of Investment Sales

Selling an investment you’ve held for under a year means your gains get taxed as ordinary income—anywhere from 10% to 37%. Hold that same asset just a bit longer, and the long-term capital gains rate drops to 0%, 15%, or 20%.​

That difference matters. On a $10,000 gain, someone earning $120,000 would pay $2,400 in short-term taxes versus $1,500 that would come from paying the long-term rates—a $900 swing based on the timing of the sale.​

“The more money you earn, the more valuable tax deductions and tax mitigation strategies become,” Harrison said. “Everything from your employee benefits to the allocation of your investments and even the structure of a small business impact tax planning.”

2. Bad Timing for Retirement Account Withdrawals

Withdrawing from a traditional IRA or 401(k) before required minimum distributions (RMDs) kick in at age 73 can often work in your favor, if you’re in a lower tax bracket during those gap years. The flip side is that waiting too long and then getting hit with larger mandatory withdrawals can push you into a higher bracket when RMDs begin.​

Financial experts often suggest tapping pretax accounts strategically between retirement and age 73 to smooth out taxable income and avoid a “tax bump” later.

3. Overlooking Tax-Loss Harvesting Opportunities

If the value of your investment drops, you can sell it to offset gains elsewhere. This strategy, called tax-loss harvesting, saved Wealthfront clients an estimated $49.83 million in 2024 alone.​

Capital losses first offset gains of the same type. Any excess can cut up to $3,000 of ordinary income, and unused losses can be carried forward indefinitely. But you have to harvest those losses before year-end.

4. Mismanaging RMDs

The years between retirement and age 73, when required minimum distributions kick in, offer a tax-planning window many people miss. Withdrawing strategically from traditional IRAs or 401(k)s while you’re in a lower bracket can smooth out taxable income and prevent a tax bump when higher mandatory withdrawals begin.

Once RMDs start, the stakes get higher. Miss the deadline, and the IRS charges a 25% excise tax on the amount you should have withdrawn. That penalty drops to 10% if corrected within two years, but it’s still money you don’t have to lose. Common errors include using the wrong life expectancy table or forgetting that outstanding rollovers affect your calculation. Your plan administrator can tell you the exact amount—just don’t forget to ask.

5. Failing To Coordinate Financial Decisions With Your Spouse

Married couples often miss planning opportunities by keeping their finances siloed. Joint filers enjoy wider tax brackets, doubled capital gains thresholds, and the ability to fund a spousal IRA even if one partner doesn’t work.​

Strategic choices—like having the higher earner max out traditional retirement accounts while the lower earner contributes to a Roth—can cut the household’s overall tax bill. But that only happens when both spouses and their advisors are talking.

Bottom Line

Tax surprises rarely come from a single decision, but can result from many small ones that add up. Before selling investments, withdrawing retirement funds, or filing jointly without a plan, loop in your CPA or financial advisor. That conversation costs little compared with a major IRS bill.