A customer account from a bank that had been insolvent for years triggered a cascading failure across hundreds of active banking systems. This incident reveals how metastable failure modes emerge when domain assumptions, service granularity, and resilience patterns interact in unintended ways.

We dissect the causal chain behind the outage, show how autoscaling and retry strategies can amplify systemic fragility, and outline architectural patterns that prevent such metastable states from forming. The analysis offers practical guidance for teams designing distributed systems that remain robust under pathological load conditions.