Elon Musk has had another bad day at the office.

The eccentric Tesla CEO sparked controversy – and added to his growing list of eccentric behaviour – by smoking weed during a live YouTube interview.

And his day went from bad to worse as his company’s stock tanked by 9% after his chief accountant stood down less than a month in the role, citing the negative publicity surrounding the car maker.

The billionaire has been plagued by controversy this week.

He doubled down on earlier, unsubstantiated allegations that the British diver involved in the rescue of the boys trapped in a Thai cave is a paedophile.

He said on Wednesday that Vernon Unsworth, who denies all claims against him, is a ‘child rapist’.

The marijuana-smoking incident took place during an appearance on the Joe Rogan Experience podcast.

The two-and-a-half hour interview, during which Musk sipped whisky and wielded a flamethrower, covered everything from artificial intelligence to his love life.

Towards the end, Rogan lit what he described as a joint containing tobacco and cannabis – which is legal in California – and offered it to Musk.

The podcast host asked his guest: “You probably can’t do this because of stockholders, right?” but Musk asked, “I mean it’s legal, right?” and accepted awkwardly, taking a puff.

During the interview, the pair discussed Musk’s ability to dream up ideas and inventions, with the billionaire comparing the way his brain works to a ‘mutation’, saying when he was a child he thought he was ‘insane’.

Musk said: “I don’t think you’d necessarily want to be me. I don’t think people would like it that much.”

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They also discussed Musk’s public persona and social media activity.

Musk admitted that he had made mistakes on Twitter.

“The vast majority of negative comments, I just ignore them,” he said. “Every now and again I get drawn in and it’s not good, you can make some mistakes.”

Musk described the “negativity” on Twitter, and later added: “Be nicer to each other. Don’t assume someone is mean. It’s easy to demonise people.”

But the most noted part of the interview was the Tesla founder’s drag on the joint, with many seeing the move as the latest in a long list of eyebrow-raising behaviour.

Musk denied he was a regular weed smoker, likening it to ‘a cup of coffee in reverse’.

Chief Accounting Officer Dave Morton’s departure comes as Tesla faces the prospect of a Securities and Exchange Commission investigation into Musk’s aborted plan to take his company private.

He is the latest in a long list of executive to have left over the past two years.

“Since I joined Tesla on August 6th, the level of public attention placed on the company, as well as the pace within the company, have exceeded my expectations. As a result, this caused me to reconsider my future,” Morton was quoted as saying in a company filing.

Tesla shares, which were down 1 per cent after Musk’s appearance on the Rogan podcast overnight, fell another 7 per cent after Morton’s resignation.