never event

Short definition from Wiktionary

Noun

1. 1
   A bad event that should never occur.
2. 2
   A bad event that should never occur: A serious adverse medical event that should never occur and thus is, by strict definitions of preventability, always preventable, being unambiguous, clearly identifiable and measurable, and resulting in death or significant disability.
3. 3
   A serious adverse medical event that should never occur and thus is, by strict definitions of preventability, always preventable, being unambiguous, clearly identifiable and measurable, and resulting in death or significant disability: A particularly shocking preventable medical error that should never occur.