One of the strangest episodes in last year's massive cryptocurrency boom was when boxer Floyd Mayweather posted an Instagram endorsement for a little-known cryptocurrency called Centra. In April, Centra's founders were indicted for fraud, with the SEC saying that many of their claims were "simply false."

In the summer of 2017, Mayweather wrote in an Instagram post that he was "spending bitcoins and ethereum and other types of cryptocurrency in Beverly Hills with my Titanium Centra Card." He urged his millions of followers to "join Centra's ICO on Sept. 19th."

Another Mayweather post promoting a different cryptocurrency said "You can call me Floyd Crypto Mayweather from now on."

On Thursday, the SEC revealed—to no one's surprise—that Centra had paid Mayweather $100,000 to promote Centra on social media. Federal securities laws require celebrities to disclose payments they receive for making these kinds of endorsements—which Mayweather failed to do.

Mayweather was paid another $200,000 to endorse two other ICOs, according to the SEC. Under a settlement with the SEC, Mayweather will disgorge his $300,000 in earnings to the SEC, as well as a $300,000 penalty and interest.

Another celebrity cryptocurrency endorser, music producer DJ Khaled, was paid $50,000 to endorse Centra, the SEC says. He will pay that $50,000 to the SEC along with a $100,000 penalty and interest.