There’s a long history of ‘easter eggs’ – hidden images, modes or features in modern technology – and synthesizers and electronic music gear are no exception.
This video, via tarekith, demonstrates the hidden pong game of the Roland MC-707 groovebox.
tarekith notes that the video is a “Quick demo to show how you can access the hidden pong game on the Roland MC707. (psst! enter demo mode and press C1 and C.”