A machine with complex servo systems runs a parent or master servo motor, by which all other motors must home to or coordinate their counted positions off of. The whole shabang has to be then processed with a very high speed processor and servo control. The most common today are the Allen Bradley RS logix 5000 and their Kinetix drives. In the past, systems were proprietary to the robot manufacturers and not real end user friendly to make any changes without enormous costs. Now, anyone in house can do it. That means jobs!!!!
The challenge is to define what constitutes an "error" in any given circumstance and to shut down the system safely when such errors occur.
The complexity of understanding all the failure modes of a robotic system coupled with the high amount of energy available to the mechanism is what has made it difficult to have robots in the vicinity of humans.
Highly complex systems with many of the same characteristics as robots fail frequently, the most notable examples being the space shuttle and other aircraft. Despite best efforts, these systems fail.