They are known as “never events”—the kind of mistake that should never happen in medicine, like operating on the wrong patient or sewing someone up with a sponge still inside—yet new research suggests that they happen with alarming frequency.

Surgeons make such mistakes more than 4,000 times a year in the U.S., according to a study led by Johns Hopkins University School of Medicine, published online in the journal Surgery. The study, using data in the National Practitioner Data Bank, a federal repository of medical-malpractice judgments and out-of-court settlements, looked at cases involving leaving an object inside a patient, wrong-site surgeries, wrong procedures and wrong-patient surgeries.

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