Pilots do have to monitor the behaviour of the autopilot. Even though they are programmed to do one thing (hold a course or altitude, change to a course or an altitude with a certain rate of change), they rely on potentially faulty sensors and control many parameters to meet the goal. Usually the autopilot will simply disconnect if it detects a problem, but there are examples of autopilots causing passenger injuries: https://en.wikipedia.org/wiki/Qantas_Flight_72