Finding the real failure behind a misleading financial API error.

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.