for as long as there have been doctors performing surgery. Bad handwriting, too much of a hurry, no one checking names against arm bands, not checking unique ID numbers to make sure that J. Smith on the operating table to have his Gall bladder removed is not the J. Smith that's come in to have her shoulder worked on.

It will never be zero, because people do stupid things and stupidity (either personal or institutional) leads to someone having the wrong leg amputated.