Bipedal balance and legged locomotion
A humanoid robot standing on two feet is a textbook underactuated system: its overall body position and orientation are far higher-dimensional than what its leg joints can directly command, especially in the direction of tipping over, which has no actuator at all. Balance has to come from how foot placement and joint torques interact with the robot's whole-body dynamics and momentum, not from any single motor pushing the body upright.
The same logic extends to legged locomotion more broadly. A running or hopping robot spends part of every stride with its foot off the ground entirely, which means for that portion of the stride its body is, in a very literal sense, an unactuated projectile subject only to gravity. Every gait has to be planned and controlled with that unpowered phase built in, not treated as an afterthought.