# Finding the real failure behind a misleading financial API error

Case 01 · Production debugging · Fintech · Jordão Qualho, Senior Software Engineer

> On a financial platform with 7M+ active users, customers were blocked in onboarding by a provider error that did not match the original failure. I traced the request path and isolated the first error.

Technologies: Node.js, TypeScript, AWS, CloudWatch, External APIs, Financial integrations

## Context

I worked on a high-volume financial platform serving more than 7 million active users. Account creation depended on a downstream financial provider.

## Problem

A subset of customers could not complete account creation. The provider first rejected some requests because of an invalid payment due-date rule. Later retries returned “Customer Already Exists”, hiding the original error and sending the team down the wrong path.

## Investigation

1. I traced the production request flow in CloudWatch logs and request tracing.
2. I compared successful and failed requests to see where their behavior diverged.
3. I worked with the external provider to understand why retries returned a different response from the first request.

## Root cause

The frontend allowed unsupported due-date values to reach the downstream integration. The provider accepted due dates only up to day 28. After the first failure, retries produced a different, misleading error.

## My contribution

- Traced the request flow and identified the original failure.
- Isolated the external integration behavior.
- Helped create a recovery path for affected accounts.
- Helped stop invalid due dates from reaching the integration.

## Outcome

Invalid due dates stopped reaching the provider, and affected accounts had a recovery path.

## What I learned

> Retries can hide the original failure. If the downstream error changes on the second attempt, the first response is still the one that matters.

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Canonical: https://jordaoqualho.com/work/financial-onboarding-incident/ · Full profile: https://jordaoqualho.com/llms-full.txt · Contact: jordaoqualho@gmail.com