Elon Musk's mean Twitter persona

The on-stage Musk, in 2015 (Photo: Justin Sullivan / Getty Images)

Until last week, it's safe to say that very few people had heard of Jarrett Walker, a public transit consultant from Portland. Then, in a piece at Citylab, Walker had the temerity to take on Tesla CEO Elon Musk for his recently stated aversion to public transport with other people. "You're an idiot," Musk tweeted in response — then, "Meant to say sanctimonious idiot."

Why it matters: Musk is the picture of Silicon Valley smooth at his electric car unveilings and space program announcements. But not so much on Twitter, where he — one of the world's most respected entrepreneurs — can seek to humiliate people who challenge him or, in his view, cast him in an adverse light.