The appealing feature of tempered glass is that it is supposed to not break into giant jagged shards that can injure you, but it’s not supposed to shatter in the first place. That’s what makes it an appealing material for, say, shower doors. Yet there’s a rare and terrifying problem: shower doors spontaneously shattering, sometimes while a person is showering, resulting in wet, naked, injured people.

That happened to a nine-year-old boy interviewed by CBS Chicago, who was taking a normal shower when the tempered glass door suddenly disintegrated, cutting him. “It just exploded. It just shattered like a bomb,” he said. “I was shaking. It was really scary.”

You can find similar stories on the Consumer Product Safety Commission’s searchable defective-products database, where adults were just as scared. One person reported pieces of glass had flown up to six feet away from the shower.

Fortunately, this seems to happen in most showers while they’re not in use, but it would be better if it didn’t happen at all. The boy in Chicago had to go to the emergency room, with lacerations from big pieces of flying glass on his arm and leg.

One glass expert recommends putting a protective film over tempered glass surfaces to prevent them from exploding at you. This is best done by someone who knows what they’re doing: hire a professional to install the film, maybe at the same time that you have the door installed.

What experts don’t know is what is causing the doors to explode.

Local 2 Investigators: Boy, 9, Injured After Glass Shower Door Shatters [CBS Chicago]

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