The Willis Tower's Ledge is a series of four glass enclosures that jut out from the building on the 103rd floor -- 1,340 feet above the ground.
When Alejandro Garibay and three relatives were on the attraction, it appeared to show cracks so they alerted staff members.
"I walked them over so they could see and they were totally shocked and asked us to step away and then proceeded to start calling staff and techs and I don't know who else." Alejandro Garibay wrote in an email to NBC 5 Chicago. "When we pulled our phones to start recording and take pictures they asked us to leave right away."
The boxes are made of three layers of half-inch thick glass and are designed to hold five tons.
According to Willis Tower spokesman Randy Stancik, it was just one layer of glass that broke and there was never any danger.
"[The top layer of the Ledge] is designed to crack when somebody drops something on it," Stancik told DNAinfo Chicago. "It's not structural. The top piece is protective."
The layer is being replaced on Thursday.
"It's perfectly fine, you can go on it now," Stancik said. "The top piece did its job. It's a scratch piece for all practical purposes. But nobody wants to go on it."