The $400 Bug My AI Coding Agent Left in Production (And Why I Didn't Catch It)

I was flying high. In less than 48 hours, I had built a full-stack SaaS scaffold using Cursor IDE and v0. The UI looked crisp, authentication worked, and Stripe checkout triggered cleanly in local testing. I launched on Hacker News on a Tuesday morning, grabbed a coffee, and waited for signups.

By noon, 35 users had signed up. By 4 PM, I checked my Stripe dashboard and realized something was catastrophically wrong: 12 checkout sessions had completed, but zero user accounts had been provisioned in my database.

I had lost roughly $400 in direct launch revenue, but worse—I had handed 12 paying customers an empty loading spinner on their very first interaction.

The Illusion of Green Checkmarks

When I pulled up my server logs, I found the culprit buried inside my webhook handler. When I prompted Cursor to generate my Stripe webhook handler, it generated textbook TypeScript code. But underneath the hood lay a classic AI coding agent trap:

```typescript // The code Cursor generated for my webhook route try { const event = stripe.webhooks.constructEvent(payload, sig, secret); await handleCustomerSubscription(event.data.object); return NextResponse.json({ received: true }); } catch (err: any) { console.log("Error processing webhook:", err); return NextResponse.json({ received: true }, { status: 200 }); } ```

Notice the bug? When handleCustomerSubscription threw a database connection timeout, the catch block logged the error to standard output, swallowed the exception, and returned an HTTP 200 OK back to Stripe.

Stripe assumed the event was processed successfully and never retried the delivery. Meanwhile, my user was left in limbo because my backend quietly reported success while failing internally.

Why LLMs Default to Swallowing Errors

AI models are trained to produce code that "works without crashing." When an LLM generates a try-catch block, its primary objective is preventing an unhandled crash in the immediate scope. Returning a 200 response inside an HTTP route handler satisfies the compiler and passes basic lint checks, but breaks distributed system guarantees.

Here is how we refactored the webhook handler to enforce proper error propagation:

```typescript // Refactored production code with strict error boundary try { const event = stripe.webhooks.constructEvent(payload, sig, secret); await handleCustomerSubscription(event.data.object); return NextResponse.json({ received: true }); } catch (err: any) { // Report to Sentry/monitoring AND return 500 so Stripe retries webhook logger.error("Stripe Webhook Delivery Failure", { error: err.message }); return NextResponse.json( { error: "Webhook handler failed transiently" }, { status: 500 } ); } ```

Key Takeaways for AI-First Founders

• Never trust generated HTTP status codes: Inspect every try-catch block to verify error HTTP statuses (500/400) are returned when database operations fail.
• Verify webhook retries: Test database connection failures explicitly to confirm third-party webhooks (Stripe, Supabase, Auth0) trigger retry queues.
• Audit before launching: Use automated AST static scanning to catch swallowed exceptions before you open your gates to real traffic.