Principle
Past success does not make a deviation safe
Nothing bad happened is evidence about the past, not automatic permission for the next run.
When it fits
- A team has operated outside a stated limit, skipped a control or used an exception repeatedly without visible harm and now treats that history as evidence that the rule no longer matters.
When to avoid it
- Do not freeze obsolete rules forever. Standards should change when better evidence and design justify change; the target is silent drift, not legitimate improvement.
Why it matters
Keep the approved boundary and the observed history separate. Repeated success outside a rule can reveal that the rule is conservative, but it can also create normalization of deviance: the absence of failure quietly becomes the new standard without a real risk review. If the boundary should change, change it deliberately with evidence, ownership and updated controls instead of letting habit rewrite it.
An example
A data migration has skipped one reconciliation step five times without an incident. That history may justify studying whether the step can be redesigned, but it does not silently delete the reconciliation requirement.
Check your result
The team can point either to the still-active original boundary or to a deliberately reviewed replacement; repeated success alone did not become the approval mechanism.
Keep this limit in mind
- Do not freeze obsolete rules forever. Standards should change when better evidence and design justify change; the target is silent drift, not legitimate improvement.
Connected ideas
Useful withReview a risky success, not only a visible failure
Evidence and sources
NASA describes normalization of deviance as a gradual shift in which unacceptable practices or standards become accepted after repetition without catastrophic results.
The concept explains one organizational failure pattern; it does not prove that every local deviation is unsafe.
Best Practices for Organizational Resilience in the International Space Station (ISS) Program · Abstract and Appendix B theme 6
NASA safety guidance explicitly warns against using past success to redefine acceptable performance.
A formal standard can still be revised when new evidence justifies it; the warning is against silent redefinition through repeated success.
The Cost of Silence: Normalization of Deviance and Groupthink · Recommendations
NASA's EVA 23 retrospective describes a recurring sensor failure being accepted as normal based on previous experience, contributing to missed significance when conditions changed.
This is one high-stakes case and should not be used to estimate risk in unrelated domains.
10 Years Ago: EVA 23 – How A High Visibility Close Call Cut Short a Spacewalk · Normalization of deviance discussion