Vested stock, and the withholding gap underneath it
When restricted stock vests it is taxed as wages, and most payroll systems withhold on it at a flat supplemental rate rather than at the rate you actually pay. For a household in a higher bracket that flat rate lands short, sometimes considerably, and nothing flags it until the return is prepared. The fix is arithmetic done in advance rather than a surprise in spring — and it is a conversation worth having the year the grant vests, not the year after.