Definition of "stumble upon"

During the course of his looking into Abedin’s S.G.E. status, the senator stumbled upon an O.I.G. “criminal” inquiry, commenced in October 2013, about whether Abedin knowingly got paid for hours she did not work while she was on vacation and maternity leave.

2016 January 31, “Is Huma Abedin Hillary Clinton’s Secret Weapon or Her Next Big Problem?”, in Vanity Fair, retrieved 21 January 2016