Every so often, the calendar and the tax code fall out of step, and ordinary workers are left to absorb the difference. New Zealand teachers are experiencing this now: an extra fortnightly pay period in the 2025-2026 financial year has produced unexpected tax bills, because the PAYE system was never designed to account for a 27-pay-period year. It is a reminder that even the most routine systems carry hidden assumptions — and that when those assumptions break, the cost tends to land quietly in someone else's letterbox.