3. Magic
It might be completely irrelevant that Scarlet Witch is due to make her first Marvel Cinematic Universe appearance in Avengers: Age of Ultron (she will be played by Silent House star Elizabeth Olsen), but it could also be due to the fact that she has some involvement in Ultron's creation. There have already been robot-like entities with magical properties in the Marvel Cinematic Universe - the Asgardian Destroyer Armour appeared in Thor - and Ultron could be something similar to that. Perhaps a novice Scarlet Witch attempts to enchant some armour and it all goes wrong? It seems like a very unlikely scenario, but judging by some of the changes from the source material in the Marvel Cinematic Universe so far, it's not one that I'd be prepared to completely rule out.