# I Created a Payment Bug Challenge… And AI Missed the Double Charge Risk 💸

Payments look simple. Check → process → done. But in real systems, this is one of the most dangerous areas. So I created a small challenge on VibeCode Arena. And things got interesting. 🚨 The Problem The logic seems correct: Check if payment already processed Deduct balance Mark as processed But here’s the issue: This system is not safe. 🧠 What Can Go Wrong? In real-world systems: Network retries can trigger duplicate requests Two requests can run at the same time Payment can be processed twice Which means: 👉 Users can get charged twice. 🤯 What I Observed When AI models tried this: Some handled basic checks Some missed concurrency issues Some ignored retry scenarios Very few understood idempotency properly The code works. But the system is not reliable. 🔥 Try It Yourself I created this challenge to test real payment system thinking. 👉 Try it here: https://vibecodearena.ai/duel/3f2646a4-88af-41a0-a44c-893e671ea1c9 Can you: Prevent duplicate payments? Handle retries safely? Design a faul