Four years ago, Tesla Motors CEO Elon Musk invited a fan to his California home and let him take his electric battery-powered Roadster sports car for a spin. His guest: Akio Toyoda, president of the world's biggest automaker. The two hit it off, and within weeks, Toyota Motor agreed to buy a $50 million stake in Tesla and sell its shuttered auto factory in California to its new partner for the bargain price of $42 million. Today the ties are unraveling, with production of the co-developed RAV4 electric vehicle scheduled to cease by yearend. Just because two companies are successful doesn't mean when they come together, they will succeed, says Ashvin Chotai, managing director of researcher Intelligence Automotive Asia. When you've got somebody threatening the status quo in an industry and they try to cooperate with the biggest player, it's bound to lead to so many complexities. Yet neither company has given up on partnerships. Toyota is co-developing a sports car with BMW, and Tesla is delivering batteries and motors for Daimler's electric Mercedes-Benz B-Class hatchback.