Glossary
Least Astonishment
Least astonishment holds that a system should behave the way a reasonable person familiar with it would expect, and that a surprising outcome is itself a defect worth fixing, independent of whether the surprising outcome happens to be safe in that particular instance. An access grant that's wider than what the requester thought they were asking for, or a permission that silently persists past when someone assumed it would end, both violate this even when nothing was technically misconfigured.
This is different from psychological acceptability, which is about a control being bearable enough that people actually go along with it. Least astonishment is about predictability specifically: even a control people are perfectly willing to follow can still violate it if what actually happens doesn't match what a reasonable person following it expected to happen.