Tesla CEO Elon Musk arrived at a Los Angeles courthouse on Tuesday for the first day of a defamation trial against British cave explorer Vern Unsworth.
Unsworth was part of a team hailed as heroes for rescuing 12 boys trapped in a flooded cave in Thailand last year.
At the time, Musk offered the use of a mini submarine.
Unsworth later told CNN that offer was a PR stunt.
Musk lashed out at Unsworth on Twitter, calling him "pedo guy." He later apologized.
But Unsworth says the insult harmed his reputation, and wants a court to force Musk to pay punitive damages.
A jury made up of three men and five women will consider whether Unsworth is entitled to damages.
Unsworth's lawyer, Taylor Wilson, in his opening statement argued that the tweet was more than just a slip-up, and that Musk had no business branding Unsworth a pedophile in what should have been quote “one of the proudest moments of his life." Musk’s lawyer Alex Spiro countered that Unsworth did not act after the tweet like a man who suffered because of it.
It's not the first time Musk's Twitter use has landed the entrepreneur in trouble.
He reached a settlement with the Securities and Exchange Commission last spring after Musk stunned Tesla investors with a tweet that he was considering taking the company private.
With nearly thirty million followers, Musk's Twitter account is a major source of publicity for his Palo Alto, California-based electric car company, which does not advertise.
The defamation trial in Los Angeles is scheduled to last about five days.
Musk is expected to take the stand in his own defense.