Every AI Agent Failure I've Debugged in 2026 was an Idempotency Problem

Five real production incidents, the 25-year-old constraint that explains them all, and the three-layer architectural fix every agent team should have shipped last quarter. Summary The failure pattern looks different every time, and it is the same pattern every time. A customer gets the same onboarding email fourteen times in nine minutes. A B2B account is charged twice for one subscription renewal. An order shows up in the OMS as three orders. A support ticket is created, escalated, re-created,