Humans can draw on a vast unconscious vocabulary of movements—we know how to politely move around someone in our path, how to sense when we're invading someone's personal space. Studies at Carnegie Mellon and elsewhere have shown that people expect social robots to follow the same rules. We get uncomfortable when they don't or when they make stupid mistakes. Snackbot, another mobile robot under development at Carnegie Mellon, takes orders and delivers snacks to people at the School of Computer Science. Sometimes it annoyingly brings the wrong snack or gives the wrong change. People are more forgiving if the robot warns them first that it might make errors or apologizes when it screws up.
More on Carnegie Mellon's robot project, HERB, can be found here. HERB is pictured above.