unintentionally vs accidentally

unintentionally and accidentally both are adverbs.

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

As adverbs, unintentionally and accidentally are synonyms defined as:

unintentionally and accidentally: without intention; in an unintentional manner

unintentionally (adverb)	accidentally (adverb)
without intention; in an unintentional manner	without advance planning
	without intention; in an unintentional manner
	of a minor or subordinate nature