an undeserved catastrophe; an undeserved ill turn; an unexpected calamity/1976年唐山大地震真是一场～。The big earthquake in Tangshan in 1976 was really an un expected calamity.
无妄之灾wu wang zhi zai
uncalled-for calamity
无妄之灾wú wànɡ zhī zāi
无妄：即“毋望”，意想不到的。形容意外的灾祸。an unexpected calamity, an undeserved ill turn, an unexpected disaster