The first time it balanced on its own. (It's actually more stable with the added mass of a rider.)

More info: http://web.mit.edu/first/segway

A group of about 12 students (MIT undergrad and grad plus high school students from FIRST Robotics teams 97, 2043, and soon-to-be-numbered Wayland HS rookies, and The Education Cooperative, have taken on the summer task of constructing a functional Segway-like scooter. Sponsored by The Edgerton Center, the D.I.Y. Segway is both a fun summer project and an educational experiment for hands-on engineering curricula.