Failed billing events are one of those classic "silent money leak" problems — same pattern shows up in D2C refunds and vendor invoice pipelines I've automated for Indian SMBs. The fix that worked for us: a dead-letter queue with a visible age metric, plus an automated daily reconciliation script that compares events emitted vs events processed. Anything in DLQ over 24h pings the ops channel. Cheap to build, kills the "unknown unknowns."