Elon Musk got Tesla’s design lead to throw a metal ball at one of the pickup truck’s armored windows. “Oh my f------ God,” Musk exclaimed as the ball smashed the glass. The price of Tesla’s Cybertruck starts at $39,900.

Tesla unveiled its futuristic pickup truck at a promotional event in Los Angeles, but when it came to showing off the vehicle’s durability, things didn’t quite go as planned.

CEO Elon Musk claimed the car was “bulletproof” against a 9mm handgun. But when he got Tesla’s chief designer to throw a metal ball at one of its armored windows, audible surprise could be heard as the glass smashed — twice.

“Oh my f------ God,” Musk exclaimed when it first happened. “Well, maybe that was a little too hard.”

Design lead Franz Von Holzhausen gave it another shot, this time with another window. Again, the ball cracked the glass pretty badly.

This was after the executive had already taken a sledgehammer to the side of the truck, striking it multiple times without leaving a scratch.

“We threw wrenches, we threw everything even literally the kitchen sink at the glass and it didn’t break. For some weird reason, it broke now,” Musk said, drawing laughs. “I don’t know why. We will fix it in post.”

It’s not clear whether the windows were supposed to shatter in the way that they did. But it’s apparent that Musk wasn’t expecting it, judging from his reaction.

On Friday, Musk tweeted that the demo had gone off without a hitch in rehearsal:

Known as the “Cybertruck,” the all-electric pickup is Tesla’s sixth vehicle since the company was founded in 2003. The vehicle’s price starts at $39,900.

This isn’t the only glass to give Tesla trouble. In 2016, Musk unveiled glass solar roof tiles to drum up support for the acquisition of SolarCity by Tesla.

The Solarglass roof tiles are meant to be an appealing alternative to bulky solar panels. However, Tesla has yet to mass-manufacture its Solarglass.