Tesla's bumpy ride continues. The National Labor Relations Board has filed a complaint against the electric-car maker, accusing it of violating U.S. labor laws when Elon Musk tweeted a suggestion in May that employees would lose their stock options if they unionized.

That tweet drew criticism at the time, as Musk seemed to say Tesla workers could forget about their stock options should a United Auto Workers effort to form a collective bargaining unit among workers at a Tesla plant in Fremont, California, succeed.

The United Auto Workers filed an objection with the NLRB the next day, leading the agency to file a complaint last week that Musk's words amounted to a violation of the National Labor Relations Act by "restraining and coercing employees" taking part in protected organizing activity.

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What did Musk say that started this dustup? "Nothing stopping Tesla team at our car plant from voting union," he tweeted on May 21. "Could do so tmrw if they wanted. But why pay union dues & give up stock options for nothing? Our safety record is 2X better than when plant was UAW & everybody already gets healthcare."

Nothing stopping Tesla team at our car plant from voting union. Could do so tmrw if they wanted. But why pay union dues & give up stock options for nothing? Our safety record is 2X better than when plant was UAW & everybody already gets healthcare. — Elon Musk (@elonmusk) May 21, 2018

Not long after the UAW made its case to the NLRB, Musk attempted to explain his thinking, saying his point was that the union was unlikely to let employees hold stock in Tesla.

Exactly. UAW does not have individual stock ownership as part of the compensation at any other company. — Elon Musk (@elonmusk) May 23, 2018

On Wednesday, a Tesla spokesperson reiterated that sentiment in an email to CBS MoneyWatch, saying "Elon's tweet was simply a recognition of the fact that unlike Tesla, we're not aware of a single UAW-represented automaker that provides stock options or restricted stock units to their production employees."

Tesla was already defending itself against NLRB allegations of labor violations at the Fremont plant, and now lawyers at the agency are seeking to consolidate the stock-option complaint with the earlier case, which is scheduled to resume Sept. 24.

Musk's social-media habits have ruffled multiple feathers, with the eccentric billionaire's more recent tweet about taking his company private fueling an investigation by the Securities and Exchange Commission and multiple lawsuits by investors.