incidentally vs accidentally

incidentally and accidentally both are adverbs.

Word	Noun	Adjective	Verb	Adverb
incidentally	No	No	No	Yes
accidentally	No	No	No	Yes

As adverbs, incidentally and accidentally are synonyms defined as:

incidentally (adverb)	accidentally (adverb)
introducing a different topic	without advance planning
of a minor or subordinate nature	without intention; in an unintentional manner
	of a minor or subordinate nature