BOSTON -- A Boston hospital has admitted that one of its experienced surgeons operated on the wrong side of a patient this week.

Officials at Beth Israel Deaconess Medical Center disclosed the mistake to staff in an e-mail on Thursday that called the incident a "horrifying story."

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The hospital said the middle aged patient was expected to have short-term discomfort, but does not have permanent organ damage or life-threatening problems.

State authorities are investigating the error, which occurred Monday during an elective operation which a state health regulator described to The Boston Globe as an orthopedic procedure.

Dr. Kenneth Sands, the hospital's senior vice president of healthcare quality, declined to provide specifics about the operation. The hospital also did not disclose the identity of the patient or surgeon, citing patient confidentiality.

State figures show that in the first five months of the year, hospitals in Massachusetts reported five wrong-sided surgeries. About 15 such errors are reported annually, said Paul Dreyer, director of the state's Bureau of Health Care Safety and Quality.

Sands said the error came during a hectic day, and the memo described the surgeon as "distracted by thoughts of how best to approach the case" just before the surgery.

Sands said medical workers marked the side of the patient that should have been operated on, but that the surgeon didn't notice the mark.

"I think he began prepping without looking for the mark and, for whatever reason, he believed he was on the correct side," Sands said.

The medical team also failed to conduct a "time out," which is a routine safety procedure that requires the operating team to call out, "Right patient, right procedure, right location."

The mistake was discovered when the patient was in the surgical recovery area. The patient, who has since left the hospital, was told about the error after waking up.

"The surgeon talked to the patient and gave a full explanation and a full apology," Sands said.

The hospital could face sanctions, depending on the findings of a state investigator.