사고가 · 사고를
accident, incident
Example
어젯밤 고속도로에서 큰 사고가 있었어요.
There was a big accident on the highway last night.
Word parts
사 matter+고 mishap
Homonym warning: a different 사고 means 'thinking'; context makes it clear.
Good to know
Accidents "happen" with 나다, not 하다: 사고가 났어요.