accidental
adjective/ˌæksəˈdɛntl/ 
happening by chance; not planned
a verdict of accidental deathI didn't think our meeting was accidental—he must have known I would be there. accidentally/‑ˈdɛntli; ‑ˈdɛntl•i/
adverbAs I turned around, I accidentally hit him in the face.The damage couldn't have been caused accidentally.