In 2020, a New Zealand woman in her 20s underwent a standard Caesarean section to give birth. The procedure seemed to go fine, but for months afterward, the woman suffered chronic abdominal pain. She visited her primary care physician multiple times because of it, and even made a trip to her local hospital’s emergency room when the pain was particularly severe.

A year later, in 2021, an abdominal CT scan finally revealed the cause of her distress: a plastic wound retractor about the size of a dinner plate had been left inside her, where it remained for 18 months. A subsequent investigation revealed that during the C-section, the surgeon had requested two retractors of varying sizes, the second (and larger) of which was left behind.

In a similar case revealed in 2018, a 42-year-old Japanese woman who complained of abdominal bloating was found to have two gauze sponges left inside her for at least six years. Likely forgotten during a previous C-section, the sponges had grown attached to the woman’s connective tissue and colon. After the sponges were surgically removed, her symptoms disappeared.

Sadly, these experiences were not isolated incidents: Surgical items are sometimes unintentionally left behind in patient’s bodies. And while medical errors may be unavoidable, leaving foreign objects inside of patients can have devastating consequences.

What Items Are Commonly Left Behind?

In England, National Health Service data from 2023 showed 291 surgical items — a record number — left hospital operating rooms inside of patients between 2021 and 2022.

Tools commonly left behind included surgical swabs, clamps, drill bits, scalpels, and, in one case, even part of a pair of wire cutters.

Read More: 6 Cases of Missing Limbs and Other Surgery Nightmares

In general, surgical items are typically left inside the abdomen or pelvis, but they’ve also been found in patients’ vaginas, chests, spinal canals, and even brains, according to a 2003 study published in the New England Journal of Medicine.

That study examined claims of “surgically retained” items filed between 1985 and 2001 with a malpractice insurer representing 22 Massachusetts hospitals. About two-thirds of the items were sponges, with the remaining one-third consisted of various instruments and tools.

When a mass of cotton is left behind, whether as a sponge or gauze, it’s even known by a specific medical term: gossypiboma.

What Are the Consequences of These Errors?

While many patients have these items removed without much complication, others are not as lucky — suffering potentially serious pain, organ damage, infection, illness, and even death.

In one case, a California man who had two surgical clamps left inside him suffered infections and septic shock that led to a stroke, according to a New York Times article. In other instances, sponges left in patients were mistaken for tumors, resulting in parts of their intestines being removed. One patient died of a brain infection after a small sponge was left inside their skull.

Read More: The Upsetting World of Primitive Brain Surgery

These errors are also a big loss for the hospitals and surgeons who make them: in the 2003 study, 47 of the examined cases resulted in malpractice litigation, with an average of more than $52,000 racked up for compensation and legal-defense expenses.

Nate Miersma, the director of medical device manufacturer Stryker, said in a 2016 Healthcare Finance article that just single surgical retention could cost a grand total of $600,000 thanks to the costs of corrective surgery and legal fees.

How Often Do Surgical Instruments Get Left Inside Patients?

While the prospect is certainly frightening to think about, in the United States, the overall odds of a surgeon leaving a foreign object in your body still appear to be pretty low.

Researchers estimated that objects may be left behind in 1 out of every 1,000 to 1,500 open abdomen and chest operations, according to the 2003 study. (Or about 0.001 to 0.0006 percent.)

Put another way, the scientists concluded that only about 1,500 of these errors happen annually in the U.S., given that there were more than 28 million inpatient surgeries in 1999.

Are These Mistakes More Common in Certain Procedures?

While nobody truly knows how frequently these mistakes occur, the researchers behind the 2003 New England Journal of Medicine study concluded that they’re more likely to happen during emergency surgeries.

Read More: What Happens to Your Brain When You’re Under Anesthesia?

What’s more, they happen despite rules and best practices to prevent such mistakes. The Association of Operating Room Nurses, for instance, has long required the use of surgical sponges that will show up on an X-ray, the study authors noted.

Sponges must also be counted, both before physicians begin a procedure and after it’s completed, and instruments are counted in all open-cavity operations. If an item goes missing, then an X-ray or even a re-opening of the patient’s cavity should occur promptly.

Still, humans make mistakes, even with checks and balances. In a 2008 Journal of the American College of Surgeons study, which examined operations performed at the Mayo Clinic in Rochester, Minnesota over a four-year period, the majority of 21 incidents had item counts recorded as correct, even though medical items were still left behind.

Could A.I. Help Prevent Medical Mistakes?

Still, scientists are turning to other avenues to prevent these types of errors. In 2021, a study by a team of Japanese researchers revealed that they’d developed an artificial intelligence program to detect retained surgical sponges.

According to the study results, the computer-aided tool eventually scored 100% for sensitivity and specificity when used to analyze X-rays of dummies known as “phantoms.” The software also achieved a 90% success rate when interpreting scans taken of cadavers with sponges placed inside them.

The effort provide hope that, as new technologies are implemented, these types of medical mistakes will happen less and less frequently in the future.

Read More: Social Media and Search Engines Can Now Forecast Disease Outbreaks