Ferris Bueller's Day Off
Made for $6M as the director's homage to Chicago, and turned into one of the best and most timeless teen comedies of all time, making its production costs back the opening weekend, racking up $70M, and becoming an iconic part of American culture. It overshadows the rest of almost every actor's career who appeared in it, and made Ben Stein the inimitable rock star of drab. Ferris' movie parents met on the set, and later married in real life. And all because John Hughes made a movie about one high school senior's sick day, and his friend who "hasn't seen anything good today".