Synonyms for Accidental (same or very similar meaning)
WordNet sense 1 (occurring or appearing or singled out by chance):
casual, chance
WordNet sense 2 (without apparent forethought or prompting or planning):
unplanned
WordNet sense 3 (associated by chance and not an integral part):
adventitious
WordNet sense 4 (without intention (especially resulting from heedless action)):
inadvertent
From the ODE community, based on WordNet
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