Apple Park, the massive donut-shaped Apple headquarters that Apple finished building last year, is an architectural marvel. The building makes extensive use of massive, floor-to-ceiling glass panels, giving the illusion that the building blends seamlessly into the surrounding forest.

But when Apple started letting employees use it in January, they discovered a big problem: they kept running into glass windows and doors. In the first few days, three people suffered injuries serious enough to require calls to 911.