Accidental, fortuitous, casual and contingent all mean happening by chance.
Accidental stresses chance or unexpected occurrence.
- I didn’t think our meeting was accidental─he must have known I would be there.
- The jury returned a verdict of accidental death.
Fortuitous so strongly suggests chance that it often connotes entire absence of cause. (see also: Fortuitous vs Fortunate )
- Their success is the result of a fortuitous combination of circumstances.
Casual stresses lack of real or apparent premeditation or intent.
- It was just a casual remark─I wasn’t really serious.
Contingent suggests possibility of happening but stresses uncertainty and dependence on other future events.
- All payments are contingent upon satisfactory completion dates.