There’s a popular saying among fans of tech billionaire Elon Musk: Never bet against him.

But in the booming world of online prediction markets, some people are not only betting against Musk, but also bringing in big paydays doing so.

On the prediction market websites Kalshi and Polymarket, some bettors are making tens of thousands of dollars by wagering that Musk will fall short on his many ambitious — but so far unrealized — plans, including a robotaxi service in California and a third political party in the United States.

Musk, the CEO of Tesla and SpaceX, has for years issued sweeping public pronouncements on social media, podcasts and earnings calls. His track record is notoriously spotty, with his unfulfilled promises about self-driving cars becoming a meme in their own right. His car service in California still has humans in the driver’s seats, and his political party idea appears dormant. And yet he also oversaw the creation of reusable, self-landing rockets and the meteoric rise of Tesla.

Now, the accuracy of his bluster is getting tested in real time as prediction markets grow in popularity.

David Bensoussan, a Polymarket user whose account is No. 51 on the site’s all-time profits leaderboard (what many call a “whale” for their sizable bets) as of Friday, said he didn’t believe Musk when, during a rift with President Donald Trump last summer, he threatened to form a new political party. So, he bet nearly $10,000 that Musk wouldn’t follow through. He made a 10% return when Musk didn’t.

Bensoussan said he’s not a fan of Musk and takes some pleasure in winning bets against those who may have been seduced by the tech baron.

“He does have a solid fan base, and so if I can help separate them from some of their money, I’m always happy to do that,” he said in a phone interview from Europe, where he’s based.

It wasn’t the only time Bensoussan bet against Musk. He correctly wagered that Musk would not buy the cable channel MSNBC after musing about the idea, that Musk would leave his White House job in May and that Tesla would fail to launch a new version of its “Full Self-Driving” software — one that didn’t require human supervision — by Musk’s target date of Dec. 31.

“He has poor impulse control,” Bensoussan said.

“He has a habit of exaggerating timelines, and of saying he’s going to do these amazing things and attaching more immediacy than what his intent may necessarily be,” he said.

That’s the opposite of the attitude prevalent among Musk’s friends and fans, who often repeat some version of: “You should never bet against Elon.” Peter Thiel, a billionaire tech investor and friend of Musk’s, used the phrase in late 2024 as Musk prepared to begin his rocky White House tenure.

Musk, the world’s wealthiest person, did not respond to a request for comment, but he has poked fun at his predictions track record.

“Now, admittedly, I’m a little optimistic sometimes,” he said at the 2024 Tesla shareholder meeting, prompting laughter from the audience. “I don’t have complete lack of self-awareness. But if I wasn’t optimistic, this wouldn’t exist — this factory wouldn’t exist.”

In total, Bensoussan has put down bets on 12 prediction markets related to Musk or Tesla and has made more than $36,000 on bets that have reached a resolution as of Friday. He mostly bets on political subjects — correctly betting that then-President Joe Biden would not resign from office, for example — and has made $1.4 million in profits all time on Polymarket, where bets are publicly disclosed. He describes himself as a “a PhD dropout” with a master’s degree in political science.

Prediction markets have enjoyed a surge in interest since 2024, when a federal appeals court cleared the way for “event contracts” such as betting on election outcomes. Polymarket and Kalshi, the two biggest platforms, have since moved further into the mainstream, announcing partnerships with Dow Jones, CNBC and the National Hockey League.

The markets have gotten more scrutiny as they’ve allowed betting on global conflicts, including Trump’s threats over Greenland. A Polymarket bet on the toppling of Venezuela’s president fueled concerns about potential trading by insiders, and the November midterm elections may create more interest in the websites.

Musk appears to be a fan of prediction markets and has posted about them frequently on X. His artificial intelligence company, xAI, has integrations with both Kalshi and Polymarket.

Both Polymarket and Kalshi prominently feature dozens of Musk-related topics at any given time, far surpassing the interest in other public figures such as Vice President JD Vance. Only Trump is the subject of more prediction markets, according to searches by NBC News. On Thursday, there were 53 prediction markets on Polymarket related to Musk or his companies and 46 on Kalshi.

The Musk-related betting topics are varied. People bet not only on whether Musk will deliver on promises like self-driving cars, but they’ll also wager on how many times he’ll tweet in a week, whether he’ll become a trillionaire this year and whether he’ll buy a major sports team.

Both Kalshi and Polymarket have employees in charge of creating new markets, although they also say that the ideas often come from the site’s users themselves.

New ways to bet on Musk pop up nearly daily. After Musk posted on X on Jan. 16 that it would be a “good idea” to buy the Irish airline Ryanair amid a feud with its CEO, both Kalshi and Polymarket started taking wagers on whether he’d follow through.

“Markets like things where there’s a lot of news going on, and more than almost anyone I can think of, on any given day, there’s almost always something going on with Elon Musk,” said Koleman Strumpf, an economics professor at Wake Forest University who studies prediction markets.

Strumpf said that prediction markets convey different information than the traditional stock market, providing a kind of reality check on all forecasts.

“What these markets potentially bring to the table is a pretty good disciplining device to people making claims,” he said.

The stock market has been forgiving of Tesla in recent years as the company has missed deadlines and experienced weakening vehicle sales. Tesla remains the world’s most valuable automaker by market capitalization, with its share price far above what its earnings alone would indicate. Over the past year, Musk has tried to reassure investors by pivoting the company toward robotaxis and humanoid robots.

But prediction markets are both more skeptical and more detailed on what to expect from Tesla. Betting on Kalshi on Friday indicated a 14.5% chance that Tesla will release its humanoid robot product, Optimus, this year.

On rare occasions, markets on Polymarket and Kalshi appear to contradict each other. On Friday, Polymarket showed odds of 68% that Tesla would launch unsupervised “Full Self-Driving” by June 30 — something Musk said he thought would be delivered by the end of 2025. But on Kalshi, a similar market showed odds of 100%, after Musk said a day earlier that he was removing human safety monitors from robotaxis in Texas.

The difference appeared to be in the fine print of how the two sites defined “unsupervised.” Video on social media showed that a driverless robotaxi was being tailed by a separate Tesla vehicle, and under the Polymarket rules, a driving mode that requires an “on-site” human to “remain ready to intervene” doesn’t qualify as “unsupervised.” The language on Kalshi was different — although on both sites, the comment sections were filled with arguments over the outcomes.

A spokesperson for Kalshi said Friday that the company had finalized the market based on reporting by Bloomberg and Reuters.

Polymarket did not respond to requests for comment. Unlike Kalshi, Polymarket does not resolve disputes on its own. Instead, the final say rests with anonymous holders of a digital token known as UMA. The tokenholders can take up to four days to vote on how to resolve a market.

Musk has helped to fuel prediction markets about him. This month, after Kalshi posted on X about Musk’s lawsuit against OpenAI, Musk responded, “What are the odds looking like?” The Kalshi account replied with a link to its website, which Musk then shared to his 232 million followers.

Once, Musk may have cost his fans money in a prediction market by weighing in. Last June, when Polymarket gave Tesla only a 14% chance of launching a robotaxi service that month, Musk posted on X that it was a “money-making opportunity.” The odds on Polymarket nearly tripled to about 40%. But the market’s rules were very specific that a robotaxi service wouldn’t count unless it were open to the general public, and the service Musk launched that month in Austin, Texas, was invite-only.

“He tweeted that it would be free money, and he didn’t even read the rules, and then he caused some people to get burned,” said Max, a Polymarket whale based in the Netherlands who followed the incident and later posted about it at length on X.

Max, who spoke on the condition that he be referred to only by his first name for privacy reasons, verified to NBC News that he has access to two Polymarket accounts with combined profits of more than $490,000. Those accounts have made dozens of bets on Musk-themed prediction markets. In one bet, wagering that Musk would leave his White House position last May, the odds were so unfavorable that he turned $363 into $5,866.

Max said he’s 23 years old and studies medicine in university while also spending three to four hours a day on Polymarket and related research. When faced with a Musk-themed prediction, he said his default position is to bet against Musk.

“There’s a lot of Elon Musk fanboys, and they tend to bet in favor of him, and if you’re a little bit more skeptical or rational, you would kind of know that his predictions are not going to come true,” he said in a phone interview.

“So I think you’re always better just betting against him, simply because he always overstates things.”