come across vs come upon

come across and come upon both are verbs.

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

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

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

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