Tesla slumps back below the key $300 level

Shares of Tesla slumped 1.6% Friday, briefly falling below the key $300 level that they've been flirting with for weeks.

Tesla got a boost early this week as CEO Elon Musk bought up $10 million worth of stock on Tuesday, two days after calling out short sellers on Twitter.

"The sheer magnitude of short carnage will be unreal," he said Friday. "If you're short, I suggest quietly tiptoeing to the exit…"

Apparently they haven't gotten the message, though. And if they have, they're sticking to their guns. Short interest — a measure of bets a stock will decline — has climbed an additional $640 million since the start of Musk's tweet-storm according to data from financial analytics provider S3 Partners.