Where did Marvel's post-credits scene tradition actually come from?

The answer

Marvel Studios head Kevin Feige has credited Ferris Bueller's Day Off (1986) as the inspiration for the MCU's post-credits tradition. In that film, Matthew Broderick breaks the fourth wall after the credits to tell audiences, 'You're still here? It's over. Go home.' Feige recalled that experiencing this as a young moviegoer was 'transformational.' The MCU's first post-credits scene appeared in Iron Man (2008), featuring Samuel L. Jackson as Nick Fury telling Tony Stark about the Avengers Initiative - a 25-second clip that launched an entire cinematic universe. While post-credits scenes existed since The Silencers in 1966, Marvel popularized them so thoroughly that audiences now expect them in virtually every blockbuster.