Dangerous Failures (or Unsafe Failures)
Dangerous failures (or unsafe failures) are failures that have the potential to prevent the safety system from achieving its safety function when there is a true demand. A single dangerous failure is generally not sufficient to prevent a redundant safety system from performing its safety function (e.g. two dangerous failures are needed for a 2-out-of-3 voting system).
Source: API STD 689, Collection and Exchange of Reliability and Maintenance Data for Equipment, First Edition, July 2007. Global Standards