"...that moron Pym..." Ha ha! I always felt it was weird that he (and the Avengers) would just throw away Ulton heads and not think of the implications. One of my earliest memories of this was way back in the 70s: Some kid found an Ultron head in a field after a battle, used it as a soccer ball, detached his "ears," got bored and tossed it aside. It was funny but you knew he was coming back and that the Avengers had made a mistake...I think this was Ultron's first appearance.