WordNet (r) 3.0 (2006):

inevitable accident
    n 1: a natural and unavoidable catastrophe that interrupts the
         expected course of events; "he discovered that his house
         was not insured against acts of God" [syn: act of God,
         force majeure, vis major, inevitable accident,
         unavoidable casualty]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

INEVITABLE ACCIDENT. A term used in the civil law, nearly synonymous with
fortuitous. event. (q.v.) 2 Sm. & Marsh. 572. In the common law commonly
called the ad of God. (q.v.) 2 Smed. & Marsh. Err. & App. 572.