Some people carry a hair tie wherever they go by wearing on their wrist like a bracelet. While it is extremely rare, a tight hair tie around the wrist could cause an infection like it did for Audree Kopp in Philadelphia. This infection didn't respond well to antibiotics so it led to surgery. If she had let it go on longer it could have led to sepsis, when an infection gets into the bloodstream.
While this kind of infection is unlikely, the result is alarming enough to make someone with long hair think twice about how they want to carry their hair ties with them.