The single biggest production-ML failure: shipping a model that scores well on the holdout set but never integrating with the rest of the system — no feature store, no monitoring, no rollback path, no incident runbook. The model works; the system doesn't. By the time the first incident hits (drift, cost spike, prompt injection), there's no instrumentation to diagnose it, no rollback to escape it, and no runbook to follow. The capstone's job is to make sure you've built all 16 of the production-hardening items before traffic hits.