happen upon vs come across

happen upon and come across both are verbs.

Word	Noun	Adjective	Verb	Adverb
happen upon	No	No	Yes	No
come across	No	No	Yes	No

As verbs, happen upon and come across are synonyms defined as:

Other synonyms of happen upon include attain, chance on, chance upon, come upon, discover, fall upon, light upon, strike.

happen upon (verb)	come across (verb)
find unexpectedly	be received or understood
	communicate the intended meaning or impression
	come together
	be perceived in a certain way; make a certain impression
	find unexpectedly