【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.