The stock price was already on the rise Tuesday lunchtime, but shortly after the tweet it jumped up from $355 to $370 -- $50 bucks short of the $420 benchmark -- but it quickly dropped back to around $362.
If Musk is being serious, it means he'd buy back the company when the market cap is $85 billion, including debt. If he does want to buy back the company at that point, he'll probably have to spend a lot more than that -- not many shareholders will sell their stock back to the company at market value.
There's always a chance this is a joke, and Engadget has asked Tesla for confirmation on that. But who knows what Musk is really thinking at any time? It's not Musk's first 420 reference on Twitter, but it's one that could lead to questions from the Securities and Exchange Commission, given the agency's guidelines on social media disclosures. Shares have since been halted.