term:

Unavoidable Casualty

unavoidable casualty n

: unavoidable accident
;also
: an unavoidable circumstance that prevents the timely performance of a procedural act (as the filing of an answer) by a party or the party's lawyer compare excusable neglect NOTE: As with excusable neglect, showing evidence of unavoidable casualty will relieve a party from a default judgment or a time limit.

Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.