At the beginning of this year, hedge funder Whitney Tilson declared he was short Tesla.
Year-to-date, Tesla shares have climbed nearly 40%.
Which is why in an interview with BenzingaTV spotted by ValueWalk, Tilson said the short was one of the worst bets of his entire career.
"Being short one Tesla, as I was, can undo a dozen good short positions when you get the occasional moonshot that runs against you," he said. "By the way, I'm no longer short Tesla — and it was one of the biggest mistakes long or shot of my investment career."
In fact, he added, only the most experienced investors should ever be short.
"Don't do it," he said. "It is exceptionally hard — I can give you a dozen good reasons why shorting is a tremendously difficult and dangerous business, and I can show you a lot of scars on my back from dozens of years of doing it myself, learning a lot of lessons the hard way."
Tilson added that his portfolio is currently 2:1 long to short, but that he's recently been "finding quite a bit more on the short side."
"I guess I would say [I am] cautious but not bearish," he said.
Here's the clip: