Surgical errors are an unexpected and terrifying occurrence for many patients throughout the U.S. A study by Johns Hopkins University found that more than 4,000 preventable surgical mistakes every year, costing more than $1.3 billion in medical malpractice costs. More than half of these mistakes cause temporary injury, approximately one-third result in permanent injury, and approximately 6.6 percent are fatal.

Patients must give informed consent before surgery, but surgical errors go beyond the normal risks of the surgery itself. Here is a look at six major surgical errors that can open up the hospital to liability:

Surgery on the wrong body part

Wrong-site surgery is known as a “never event” because doctors and medical experts acknowledge that, outside of negligent activity, these things should never happen. In unfortunate cases, doctors operate on the wrong body part (such as the right leg instead of the left), further complicating the patient’s medical situation.

Surgery on the wrong patient

Even more troubling is when a surgeon performs surgery on the wrong patient. In this day and age, there is no excuse for failing to properly identify a patient before surgery. Hospitals generally follow a patient identification system to make sure the correct patient is scheduled for the right treatment. These systems often require two forms of patient identifiers (such as a state-issued ID, a hospital-issued patient wristband, or verbal confirmation of birth date) before administering medication, performing surgery, or collecting patient samples. Researchers estimate that surgery is performed on the wrong patient at least 20 times per week nationwide.

Medication or anesthesia errors

Adverse drug reactions contribute to nearly 100,000 trips to the emergency room for senior citizens in the U.S., and patients of all ages have suffered from being administered the wrong medicine. Whether it is the wrong dose of a certain medicine or the wrong medication altogether, medication errors can have serious consequences. In addition, it is crucial for anesthesiologists to administer the right dose of anesthetics to ensure the patient will safely go unconscious and stay unconscious for the duration of the surgery (for general anesthesia) or the affected area is properly numbed (for local anesthesia).

Leaving foreign objects in a patient

Several thousand patients are injured every year by surgeons or assistants leaving a foreign object inside the patient, such as a scalpel, sponge, clamp, or surgical scissors. (In approximately two-thirds of cases, the foreign object is a surgical sponge.) These objects may not be noticed until they start to wreak havoc inside the body, causing infections or other adverse reactions.

Post-surgical infections

Approximately 1 in 20 patients develop an infection while hospitalized, contributing to nearly 100,000 deaths. These infections can be caused by poor hygiene on the part of the medical staff, foreign objects left in the body during surgery, or other factors.