Musk, who also runs SpaceX and a number of other companies, has been known to make big often exaggerated promises. For example, he once called the Hyperloop, which was never built, the "fifth mode of transport" and famously promised to take Tesla private in a Twitter post, saying he had secured financing that he didn't have.

Musk, who last year said “things that inhibit freedom of exchange or distort the market are not good,” became $12 billion poorer Monday.