It's Saturday 19:42 WAT. You're the on-call engineer at a card-acquiring fintech. Your payment provider (Paystack, Flutterwave, Stripe, pick your poison) just made a routing change on their end. The unintended consequence: their webhook delivery queue, which normally trickles in real-time events to your /webhook endpoint, just started replaying the last 72 hours of deliveries in a 90-minute burst. Your endpoint is getting ~12,000 webhooks per minute, every one of which is a re-delivery of an event you already processed days ago. If your handler is idempotent, this is a load spike. If your handler is NOT idempotent, this is a double-booking catastrophe in real time. Walk it as the engineer who finds out which one you shipped.