unexpectedly vs accidentally

unexpectedly and accidentally both are adverbs.

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

As adverbs, unexpectedly and accidentally are synonyms defined as:

Other synonyms of unexpectedly include by chance, circumstantially.

unexpectedly (adverb)	accidentally (adverb)
without advance planning	without advance planning
in a way that was not expected	without intention; in an unintentional manner
	of a minor or subordinate nature