An experienced surgeon at Boston-based Beth Israel Deaconess Medical Center recently operated on the wrong side of a patient, a mistake disclosed in an e-mail that hospital administrators sent to staff members. Massachusetts authorities are investigating the errant surgery, which happened during an elective procedure. The mistake happened as hospitals, regulators, and insurers are devoting unprecedented attention to combating medical errors. In June, Massachusetts said it would stop reimbursing hospitals for medical costs associated with mistakes.