When Elon Musk unveiled the Tesla Cybertruck last week, things didn’t go according to plan when lead designer Franz von Holzhausen tested the durability of the Cybertruck’s “armor glass.” He managed to smash two of the vehicle’s windows onstage with a metal ball, soon after smacking the door with a sledgehammer (unlike the glass, it was fine). We have now learned that, according to Musk, it was this sledgehammer impact that damaged the glass, which is why the windows subsequently smashed when hit by the ball. The Verge reports: This seems plausible, especially as Musk also shared a slow motion video of von Holzhausen performing the same exact test before the event, with the ball bouncing harmlessly off the window. The combined impacts likely weakened the glass, setting the stage for the eventual smash. (Though why the back window broke as well isn’t clear: the passenger door didn’t get whomped by the sledgehammer.) At any rate, the smashed glass was just one moment in an event which gave viewers plenty to talk about without the on-stage mishaps. The divisive design and impressive specs of the Cybertruck have caught the world’s attention, and since the unveiling Musk has been drip-feeding bits of information on Twitter to keep people engaged.