happen upon vs come upon

happen upon and come upon both are verbs.

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

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

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

happen upon (verb)	come upon (verb)
find unexpectedly	take possession of
	find unexpectedly