Kestrel Bay Retirement Advisors sees the same RSU mistakes to avoid again and again, including a wrong 1099-B basis, flat withholding that falls short, and selling at a loss just before a new vest. RSU income is taxed as ordinary wages at vest on that day's share price, and the same value is your cost basis, so a 1099-B showing a lower basis means you can pay tax on the same dollars twice.
Gwen and Sam, a hypothetical couple aged 52 and 53, learned this in the year her stock released $2,000,000 of shares. Their W-2 looked fine, their broker's form did not match it, and a loss they had planned to harvest was pushed into a later year. None of it was exotic. Each item was a default setting nobody had checked.
The Kestrel Bay Retirement Advisors team wrote this article for people with several years of vests behind them. We go through each error with its dollar cost, then the better way, then a checklist you can run against your own paperwork.
Which RSU mistakes cost the most money?
Ranked by dollars, the biggest is simply holding everything after it vests. A 30% drop on a $1,000,000 position is $300,000, and no tax trick comes close to that. Next comes flat withholding that is too low: $75,000 short on a $500,000 vest. Then a blank basis at $20,000 of extra tax, and a wash sale at $9,000 of tax pushed out. The table shows all four side by side, and the last three are the ones people fix with paperwork alone.
Take the withholding gap. Say 22% was withheld on a $500,000 vest, but Gwen and Sam's income is already above $768,700 (married filing jointly, 2026), so the extra income is taxed at 37%. The difference is 15 points, and 15% of $500,000 is $75,000, due when they file. Withholding has its own page on this site, so we'll keep it short here.
Missing the safe harbor can make it worse. That means paying in at least 100% of last year's tax, or 110% if last year's AGI was over $150,000 (check the current IRS rule), and falling short can add an underpayment penalty to the tax itself. A big vest in the fourth quarter is the classic trap, because nothing was paid in earlier.
Before Kestrel Bay Retirement Advisors suggests any sale, it estimates the tax bill of the sale and of the next vest. Selling a lot you didn't price first is how a good diversification move turns into a surprise in April.
| Mistake | What goes wrong | Cost |
|---|---|---|
| Blank 1099-B basis | 1,000 shares sold at $100 report $100,000 gain | $20,000 extra tax at 20% |
| Flat withholding | 22% withheld on $500,000 vest; 37% owed | $75,000 short at filing |
| Wash sale at new vest | $45,000 of loss moves into new shares | $9,000 of tax waits |
| Holding all after vest | $1,000,000 position falls 30% | $300,000 lower value |
How do you catch a double-counted basis on a 1099-B?
The most common mistake is accepting a 1099-B that shows a blank or zero basis. On 1,000 shares that vested at $100 and were sold at $100, it reports a $100,000 gain instead of $0, about $20,000 of avoidable tax at an assumed 20% rate. Brokers often report the basis wrongly because the vest income reached your W-2 through payroll, not through the brokerage. The fix takes four steps.
- Step 1: Take the vest-day price from each release confirmation and multiply by shares. 1,000 shares at $100 is $100,000, and that amount already sits in W-2 box 1.
- Step 2: Find the same lot on the 1099-B and compare the basis. If it is $0, blank or lower than $100,000, the gain is overstated by the difference.
- Step 3: On Form 8949, enter the sale as the broker shows it, then adjust the basis with code B and the correcting amount, so the IRS record and your return agree.
- Step 4: Keep one line per lot (vest date, shares, vest price, sale date) in a simple sheet. Sell-to-cover shares are lots too, and they're often the ones with the wrong basis.
How does a new vest trigger a wash sale?
A vest counts as acquiring shares. So a sale at a loss within 30 days before or after a vest disallows the loss on as many shares as vest. The disallowed loss is added to the basis of the new shares, so it isn't gone for good.
Hypothetical: Gwen's earlier vest left her shares with a $120 basis. She sells 2,000 shares at $90, a loss of ($120 − $90) × 2,000 = $60,000. Ten days later, 1,500 new shares vest. The rule disallows the loss on 1,500 shares: 1,500 × $30 = $45,000. Only 500 × $30 = $15,000 counts this year. The $45,000 moves into the new shares' basis, now $90 + $30 = $120 each. At an assumed 20% rate, $45,000 × 20% = $9,000 of tax savings waits until those shares are sold.
With quarterly vests (an assumption), the 61-day window leaves only about a month open. Gwen should place loss sales mid-quarter and check the calendar first.
Yes, that means paying some tax earlier than planned. The loss is delayed, not erased, but the delay can land in a year with a bigger bill.
Pros and cons of selling RSU shares at vest
Selling at vest solves several of the problems above, but it isn't free. Here is how we weigh it with clients.
- Pro: selling on the vest date leaves almost no gain or loss after the income is taxed, and a vest that has already been sold can't cause a wash sale.
- Pro: it cuts concentration at once, which matters for Gwen and Sam when $2,000,000 of her stock released in one year.
- Con: you give up the long-term rate on any later rise, since shares sold within a year of vest produce short-term results.
- Con: the sale happens on a fixed date at whatever the market pays, and you can't pick which lots to sell or to gift.
Does the right fix depend on your age and vest size?
Yes. A small vest of $50,000 a year leaves a withholding gap of about $7,500 (15 points of $50,000), and a hand-kept lot sheet is enough. At $500,000 or more, the gap and the basis errors deserve a tax estimate before each vest.
Age matters for the first mistake. At 52, Gwen has roughly 10 to 15 working years to recover from a 30% drop, while at 38 the same drop leaves more time. The older you are, the less time you have to wait for a recovery. Investing involves risk, including loss of principal.
Income matters too. Above $768,700 (married filing jointly, 2026), every extra vest dollar is taxed at 37%, so wash sale and basis errors cost more per share there than in lower brackets. Gifting appreciated shares is a separate topic with its own page.
Checklist: what to verify before your next vest
Run these five checks once and then repeat them after every vest.
- Release confirmation saved for every vest, with date, shares and price.
- Each 1099-B line matched to a lot; basis equals the vest-day price times shares.
- Calendar marked with the 30 days before and after each vest, with no loss sale inside it.
- Withholding rate compared with your marginal rate, and an estimated payment date set if short.
- State rules checked in general form: some states tax RSU income by where it was earned.
What can you do this week about these RSU mistakes?
Start with paper, not trades. Nothing below requires selling a share, and it fits in an evening.
- Pull last year's 1099-B and release confirmations and check the three biggest lots for basis.
- Find the next vest date and write down the window 30 days either side.
- Multiply your next vest by the gap between your marginal rate and the withholding rate to see the likely shortfall.
- Decide which shares would go to a donor-advised fund and which would be sold, before the next vest changes the picture.
What questions should you ask an advisor about RSUs?
Ask these before you hand over any statements. Good answers include a number, not a reassurance.
This page covers RSUs at a public company only. ISOs, ESPP shares and double-trigger RSUs at an IPO have their own rules, and if you have never sold a vested share, the basis mistake hasn't happened to you yet. It is general education, not individual tax advice.
Kestrel Bay Retirement Advisors can reconcile your forms, price each sale and map your vest dates. Our fees are explained in a written schedule, and the request form on this site is the way to ask for a first conversation.
- Will you estimate the tax of each sale before you recommend it, and show me the number?
- Who reconciles my 1099-B against my release confirmations, and how often?
- Which lots will you sell first, and how do you keep a loss sale away from my vest dates?
- How do you coordinate sales with my estimated tax payments, and where is your fee explained in writing?
Your questions about RSU mistakes to avoid, answered
Does an RSU vest count as buying shares for the wash sale rule?
Yes. When RSUs vest you acquire shares, and that acquisition can trigger the wash sale rule if you sold the same stock at a loss within 30 days before or after the vest. The loss is disallowed on as many shares as vested and moves into the new shares' basis.
Can I still fix a wrong cost basis after I filed my tax return?
Usually yes. If the basis was too low, you can file an amended return (Form 1040-X) with a corrected Form 8949 showing the right basis, within the IRS time limit for refunds. Keep the release confirmations and W-2 as proof, and check the current IRS deadlines.
Do losses on RSU shares offset my other capital gains?
Yes, if the loss is allowed. A capital loss on RSU shares first offsets other capital gains, and up to $3,000 of any excess can reduce ordinary income per year, with the rest carried forward. A wash sale delays the loss, so check the 61-day window first.
This information is general education only and does not account for your specific circumstances, goals or finances. It is not investment, tax or legal advice. Investing involves risk, including the possible loss of principal. Before making any financial decision, consult a qualified professional about your own situation.