A Cal/OSHA "serious" violation is one where there is a realistic possibility that:

Explanation

Labor Code section 6432 defines a serious violation as one where there is a realistic possibility that death or serious physical harm could result from the actual hazard created by the violation. Notice what it does NOT require. Nobody need have been hurt (a), which is why the class attaches to an unshored trench that every worker climbed out of safely. Option (c) describes the repeat class, which turns on a substantially similar violation within the look-back period. Option (d) names the employer-knowledge element, which is real but works in the opposite direction: section 6432(b) lets the employer rebut the presumption by showing it did not know and could not with reasonable diligence have known of the violation. Knowledge is a defence to the classification, not its definition.