Sending money looks simple.
Check balance → deduct amount → done.
That’s how most basic systems work.
But what if this logic allows users to spend more than they have?
So I created a challenge on VibeCode Arena to test this.
And the results were surprising.
The Problem
At first glance, everything works:
- Check if balance is enough
- Deduct amount
- Return success
Simple.
But there’s a hidden issue.
⚠️ What Can Go Wrong?
In real-world systems:
• Multiple transactions can happen at the same time
• Each request checks the same balance
• All requests pass the condition
• Balance gets deducted multiple times
And suddenly:
Balance becomes negative
System allows overspending
Financial data becomes incorrect
Why This Happens
This is a classic race condition problem.
Each request sees the same initial balance before it gets updated.
So even if the logic looks correct…
The system is not safe under concurrency
What I Observed
When AI models tried this challenge:
- Some handled basic logic correctly
- Many ignored concurrent transactions
- Some didn’t ensure atomic updates
- Very few treated it as a financial integrity issue
The code works.
But the system is not reliable.
The Real Issue
This is not just a coding problem.
It’s about:
• Data consistency
• Transaction safety
• System reliability
• Real-world behavior
Because in financial systems:
One wrong transaction = big problem
Try My Challenge
I created this challenge to test real-world backend and fintech thinking.
Try it here:
https://vibecodearena.ai/duel/3bb0d058-5dfe-44d4-8e73-d238316cd115
Can you:
- Prevent negative balance?
- Handle concurrent transactions safely?
- Design a reliable wallet system?
Final Thought
Wallet systems don’t fail because of math.
They fail because of timing and consistency.
And that’s where real engineering begins.
Try it and tell me
Did AI handle transactions safely… or allow negative balance?