Here's how it happens to lots of people.
1. Vest RSUs worth $25k-500k, depending on how senior you are and what BigCo you work for
2. Employer sells 40% of your shares instantly upon vesting, for taxes
3. Employer remits that 40% to the government
4. You sell all your shares, because you realize concentrating your risk of investment loss with the risk of unemployment is a bad idea. Basis should be approximately 60% of that year's vesting
5. Employer correctly reports the full pre-tax vested RSU value as income on your W2
6. Broker, for incredibly Byzantine and unfortunate legal reasons, reports basis of vested-then-sold stock as zero
If you miss step 6. when filing your taxes and just naively enter the numbers your broker gives you in your tax forms, you will be double taxed, capital gains on the full value of your sale, instead of ~$0 gain for your vested-then-sold stock. Your employer already covered that income on your W2.
So, sokoloff may have just inappropriately paid capital gains on a $300k vest. Sizable, but fairly normal for a high performing senior engineer or manager at a BigCo.
Alternatively, the same logic applies (and it's even easier to screw up) if you held on to your shares from multiple years of vesting, and sold them all at once, so the original grants may have been smaller. If they appreciated, you owe tax on the adjusted basis, not the zero basis reported by the broker.
Side note: you need to check this particular failure even if you take your return to a place like H&R Block. H&R Block specifically has repeatedly screwed this up for many different coworkers of mine. And then they didn't help with the necessary amended return. Stear clear of H&R Block.
(I wish I vested $300K of shares per year... ;) )
150k of miscalculated basis would be about 250k of pre-tax RSUs, which is the ballpark I was vaguely thinking of.
I assumed in Sheets (arbitrarily) one grant, 10% CAGR on share price, annual vesting, and selling the whole grant in year 4 (so years 1-3 growth are LTCG and year 4 is no growth after vest).
https://docs.google.com/spreadsheets/d/1zY4DUwid_syt1bdCvvkc...
Our total filing packet runs to nearly 100 pages and our annual bill is between $1800 and $2500, depending on the exact complexity, whether we have any amended filings to make, etc.
I DIY'd my taxes for many years, and once the kids came and brought the additional complexity (some of which is cyclic), I tried a professional one year. Literally in the first year, he looked over my prior DIY'd returns [that I was certain I'd done correctly] and found an error in one year where I'd overpaid $29K by miscalculating the basis of some employee RSUs and it was still within the statute of limitations where I could amend that year. He saved me 10 years' worth of his fees before he'd filed my first full year of taxes.