Published onMarch 13, 2026Idempotency Issues in Payment APIs — When Retries Charge Customers Twicebackenddistributed-systemspaymentsreliabilitysystem-designnodejsmicroservicesNetwork timeout on a payment request. Client retries. Customer gets charged twice. This is the most expensive bug in fintech — and it's completely preventable with idempotency keys. Here's the complete implementation.