German engineer Sami Haddadin at the German Aerospace Centre Space Agency has designed a robot that he lets punch him on the head.

Why would someone create a robot that hits people? "Do not harm humans" is the credo we want robots to live by.
Actually this is what Mr. Haddadin tries to accomplish. He tries to make industrial robots safer by teaching them to avoid hitting people. To be able to do that the robot needs to know what it 'feels' like to hit people. In the prototype robot he put sensors in the six joints of a robot arm, and developed a program that would put a stop to movement if something unexpected happens. German robot company Kuka is supposed to launch such a 'do not punch humans' industrial robot next year.
More details on the NewScientist and on Telegraph.