Doctors were shocked to find hundreds of tiny gold needles after X-raying a South Korean woman's knee.
The 65-year-old woman had been receiving acupuncture for knee pain, according to a study in the New England Journal of Medicine.
She had been diagnosed with osteoarthritis.
It's thought the needles were left inside on purpose to provide "continued stimulation" after the treatment had ended.
But many medical experts say leaving foreign objects in the body is never a good idea.
Such objects can lead to inflammation, abscesses and infection, and also make it hard for doctors to read X-rays or perform MRIs, Dr. Ali Guermazi, a professor of radiology at Boston University, told Live Science.