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.

"I walked into a glass door on the first floor of Apple Park when I was trying to go outside," one person said in a January 4 911 call obtained by the San Francisco Chronicle. He said he hit his head but didn't suffer serious bleeding.

Two other emergency calls from Apple's campus were made two days earlier on January 2—which the Chronicle says is the day Apple employees started moving into the new headquarters. "We had an individual who ran into a glass wall pane and they hit their head," an Apple caller said on one of the January 2 calls. "They have a small cut on their head and they are bleeding, slightly disoriented."

"We had an employee, he was on campus and he walked into a glass window, hitting his head," another caller said on the same day. The caller said the middle-aged employee "has a little bit of a cut on the eyebrow," and that the injury was likely serious enough to require stitches.

Thankfully, none of the injuries seemed to be life-threatening and didn't require hospital visits.

The San Francisco Chronicle reports that a city official in Cupertino, where the headquarters is located, had warned Apple about the risk of absent-minded employees walking into glass walls and doors. Fire codes require glass walls and doors to be clearly marked for this reason.

In January, Apple vice president of real estate development Dan Whisenhunt said that Apple hadn't had too much trouble with birds flying into the building's huge glass windows. "Now the humans on the inside, that’s a different story,” Whisenhunt said, according to the Chronicle. “We’ve had people bump into the glass. That’s a problem we are working on right now.”

After that trio of 911 calls, Apple began to take this problem more seriously, adding stickers to the windows to make them easier to see. The Chronicle says that since then, "there were no other incidents in which emergency services were called to treat people running into the glass."