A withholding gap is the mismatch between what was withheld from your pay and what your tax return says you owe. When withholding falls short, you write a check in April and may owe an underpayment penalty. When it runs over, you get a refund, which means you made an interest-free loan to the government all year.
Gaps come from a few predictable places. Bonuses and RSUs are withheld at a flat 22% (2026), which undershoots anyone in a higher bracket. Two earners in one household each get withheld as if they were the only income. Side income, investment gains, and ESPP sales have no withholding at all.
A concrete case: someone earns $180,000 in salary and $120,000 in RSUs. Salary withholding is roughly right. The RSUs are withheld at 22%, or $26,400, but at a 32% to 35% marginal rate the tax on them is $38,000 to $42,000. The gap is $12,000 to $16,000, due in April, plus a possible penalty if it was not covered by estimated payments.
The common mistake is finding out in April. The cure is a mid-year check: compare year-to-date withholding on a pay stub against a rough projection of the full-year tax, and close any gap through extra W-4 withholding or an estimated payment. The IRS safe harbor, paying at least 100% of last year's tax (110% if your income was above $150,000), avoids the penalty even if you still owe the balance.